Part One: Planetary Size.
You should specify what you mean by your planet having about 1.25 the size of Earth.
If you mean that it will have about 1.25 the volume of Earth, it will have about 1.0775 the radius and diameter of Earth, and about 1.16100 the surface are of Earth.
If it will have about 1.25 the surface area of Earth it will have about 1.1180 the radius and diameter of Earth, and about 1.3974 the volume of Earth.
If it will have about 1.25 the radius and diameter of Earth, it will have about 1.5625 1180 the surface area of Earth and about 1.953125 the volume of Earth.
And if you mean that it will have about 1.25 the mass of Earth, how its radius, surface area, and volume compare to Earth will depend on the ratio of its average density to the average density of Earth. If it has exactly the same average density it will have 1.25 times the volume of Earth. But the density of planets depends on the density of the materials they contain and also their mass, since more massive planets compress their matter more through their gravity. So a planet with 1.25 the mass of Earth could have more or less than 1.25 the volume of earth, depending on its average density.
Part Two: Tidal Heating of a Satellite world.
Tidal interactions with companion worlds could help to heat up the interior of your world, including its internal oceans. I note that your world is described as:
The planet is orbited by 3 moons, all 3 are much smaller than our moon.
There will be interesting tidal effects on and world orbited by three moons which usually pull in different directions and sometimes pull in the same direction.
But if the moons are all much smaller than the Moon their total effect on tidal heating should be much less than that of the Moon on Earth, and not enough to keep all of the oceans liquid. So maybe you should increase the mass of your world's companion world or worlds to increase the tidal heating.
Objects massive enough to be gravitationally rounded and less massive than about 13 times the mass of Jupiter (or about 4,131.4 times the mass of Earth) are called planetary mass objects and are considered to be planets if they orbit stars. The natural satellites of planetary mass objects are called moons.
Objects more massive than about 75 times the mass of Jupiter (or about 23,835 times the mass of Earth) are called stars, and planetary mass objects orbiting them are called planets.
Your rogue planet in interstellar space can't be heated by tidal heating with a companion star, because it would also be heated by the radiation from the star, and it would called simply a planet instead of a rogue planet in interstellar space.
You planet might not be a planet at all. It could be a giant, larger than than Earth, moon of a giant planet that drifts in interstellar space far from any star, a moon of a rogue planet.
Objects with mass between 13 and 75 times the mass of Jupiter exist and are called brown dwarfs. Since brown dwarfs are not considered to be either planets or stars, I don't know what large, planetary mass natural satellites of brown dwarfs should be called. It may be necessary for your world to be a natural satellite of a brown dwarf, and thus be called a planoon or moonplan, or whatever, instead of planet.
Most stars in our galaxy are so dim that a planet orbiting them in their habitable zones would be so close they would be tidally locked to the star, so that one side always faced the star and the other side always faced away from the star. It is uncertain whether a planet tidally locked to its star would be habitable. But if a giant planet orbited in the the habitable zone of its planet, any moons of that planet large enough to be habitable would be tidally locked to the planet and not to the star and so would experience a normal succession of days and nights.
"Exomoon habitability constrained by illumination and tidal Heating" discuses some of the factors affecting the hypothetical habitability of moons orbiting planets in other star systems. https://arxiv.org/ftp/arxiv/papers/1209/1209.5323.pdf
It introduces the concept of the Habitable edge, a distance around a giant planet where any moon orbiting within would have some much tidal heating that it would suffer a runaway greenhouse effect. And thus the tidal heating on a moon of a giant planet can heat it up significantly. Perhaps tidal heating of a moon of giant planet orbiting far beyond the habitable zone of its star, or even the moon of a rogue planet in interstellar space, could be strong enough to make the moon warm enough for liquid water on its surface, or maybe buried beneath miles of ice.
Part Three: Internal Heat.
Worlds have other sources of internal heat besides tidal interactions with other worlds.
They have left over heat of formation. At the end of planetary formation giant asteroids will be crashing into the planet, producing enough heat to melt the surface of the planet into lava. It may take many millions of years for a new planet to cool off and become cold enough for liquid water to form on its surface.
But if you want the planet to have advanced multicelled plant and animal life in its subsurface ocean, it will have to be billions of years old, if that life evolved naturally on that planet instead of being seeded by visiting aliens. And in the billions of years it takes to evolve multicelled life the planet will cool off and lose a lot of its internal heat, and may no longer be warm enough to heat up the subsurface oceans. I note that the subsurface oceans might be liquid at very cold temperatures due to the intense pressure of tens or hundreds of kilometers of water and ice above.
And an another source of internal planetary heat would be the heat produced by the radioactive decay of unstable heavy elements. Such internal heat has only a minor effect on the surface of Earth at its present age, after billions of years of radioactive decay to non radioactive isotopes and elements. So possibly the planet in your story is very young and still has a a lot of radioactive heat, and the advanced lifeforms in the oceans could have been seeded there by visiting aliens.
I note that the interstellar medium that stars and planets form out of is enriched with radioactive materials when stars in certain stages of their development expel a lot of material. And many of the heaviest and most unstable elements are formed in supernovas and expelled into interstellar space. Thus the interstellar medium becomes richer and richer in heavy radioactive elements over time, and younger stars and planets are richer in radioactive elements.
The proportion of heavy elements in a star or planet also depends on where in the galaxy it forms, as well as when. Stars have higher proportions of heavy elements the closer they form to the galactic core, and lower proportions of heavy elements the farther they form from the galactic core.
So obviously the planets with the most heavy elements are formed in the galactic core, and your planet should either be in the galactic core or else should have been ejected from the galactic core by close encounter with a star and now orbit near to the Earth, depending on the needs of your story.
So it is possible that maybe there could be rogue planets with high enough concentrations of radioactive elements to have liquid water subsurface oceans even after the billions of years necessary for life in those subsurface oceans to evolve complex multi celled organisms. Or maybe it would be necessary for the world to also have a large companion and get a lot of tidal heating from that companion to he warm enough.
And maybe it won't be possible for a rogue world to have enough combined heating from radioactive decay and tidal heating to have liquid subsurface oceans after the billions of years necessary to evolve complex multi celled lifeforms, and the world might be only a few hundred million years old, and might have been seeded with life by visiting aliens.
Part Four: Atmosphere.
The question says:
The planet has a very thick atmosphere of primarily nitrogen and oxygen with notable amounts of carbon dioxide. (Similar to earths composition except with some extra carbon dioxide.)
And so the question seems to assume that the planet has a gaseous atmosphere instead of one frozen into exotic ices. And the question may assume that the oceans are warm enough to be liquid all he way to the top, instead of being covered with ice caps. If your rogue planet is that warm somehow you might as well assume that it has some dry land with plants and animals on the dry land, as well as sea life.
Here is a link to a list of the largest bodies of liquid in our solar system. You will note that most of them are subsurface oceans under thick layers of ice.
https://en.wikipedia.org/wiki/List_of_largest_lakes_and_seas_in_the_Solar_System
There are seven known sub surface oceans listed, plus about eleven possible subsurface oceans.
Titan is the largest world with a subsurface ocean.
Titan is probably partially differentiated into distinct layers with a 3,400-kilometer (2,100 mi) rocky center.[31] This rocky center is surrounded by several layers composed of different crystalline forms of ice.[32] Its interior may still be hot enough for a liquid layer consisting of a "magma" composed of water and ammonia between the ice Ih crust and deeper ice layers made of high-pressure forms of ice. The presence of ammonia allows water to remain liquid even at a temperature as low as 176 K (−97 °C) (for eutectic mixture with water).[33] The Cassini probe discovered the evidence for the layered structure in the form of natural extremely-low-frequency radio waves in Titan's atmosphere. Titan's surface is thought to be a poor reflector of extremely-low-frequency radio waves, so they may instead be reflecting off the liquid–ice boundary of a subsurface ocean.[34] Surface features were observed by the Cassini spacecraft to systematically shift by up to 30 kilometers (19 mi) between October 2005 and May 2007, which suggests that the crust is decoupled from the interior, and provides additional evidence for an interior liquid layer.[35] Further supporting evidence for a liquid layer and ice shell decoupled from the solid core comes from the way the gravity field varies as Titan orbits Saturn.[36] Comparison of the gravity field with the RADAR-based topography observations[37] also suggests that the ice shell may be substantially rigid.[38][39]
Titan has a dense atmosphere:
Titan's atmospheric composition is nitrogen (97%), methane (2.7±0.1%), and hydrogen (0.1–0.2%), with trace amounts of other gases.[14]
Titan's surface temperature is about 94 K (−179.2 °C). At this temperature, water ice has an extremely low vapor pressure, so the little water vapor present appears limited to the stratosphere.[64] Titan receives about 1% as much sunlight as Earth.[65] Before sunlight reaches the surface, about 90% has been absorbed by the thick atmosphere, leaving only 0.1% of the amount of light Earth receives.[66]
https://en.wikipedia.org/wiki/Titan_(moon)
At Titan's distance from the Sun, it receives only about 1 percent as much heat from the Sun as Earth does, and so it is very cold. But a rogue planet in interstellar space would receive far less light and heat from distant stars and so should have a much colder surface temperature than Titan, and so the upper layers of its oceans should be frozen solid. Unless that world has very intense internal heat.
I note that on Earth, almost all the oxygen is produced by plants via photosynthesis which requires sunlight. On a rogue planet in interstellar space there would be only extremely weak light from the stars.
So your planet would have to have another source of light, such as volcanic eruptions, bio luminous organisms (which would require some source of energy), or constant lightening in its stormy atmosphere.
It is possible that oxygen could have produced by sunlight when the rogue planet was still in a stars system before being ejected into interstellar space. Sunlight on solid or liquid water would evaporate it, putting water vapor into the atmosphere of the planet. Sunlight would break up water vapor molecules in the atmosphere into hydrogen and oxygen. The hydrogen would tend to escape from the planet, but the oxygen would be retained by a planet "1.25 times the Size of Earth".
So the planet could possibly have built up a lot of oxygen in its atmosphere while it still orbited its star, even if it didn't have photosynthesizing plants. But if animals evolved to use oxygen, they would reduce the amount of oxygen in the atmosphere.
It is possible that the organisms in the planet evolved a different biochemical source of energy as good as using oxygen, and so never evolved to use oxygen, thus keeping the oxygen in the atmosphere instead of using it up.
Anyway, those are my ideas.
