If the ship's orbit is perfectly circular and with a speed of 3km/s, it will be orbiting at an altitude of approximately 37,917 kilometers above sea level. That is just a very little bit above geosynchronous orbit.
This may be interesting for you: geosync altitude is kinda the sweet spot for communication satellites, so slots in it are in high demand. If you check the interactive graphic in this page, you will lots and lots of satellites there.
Then we have this article from Sky and Telescope:
Many geosynchronous satellites shine between magnitudes 10–12, so you can spot them in telescopes as small as 4 inches.
On top of that, being close to geosync altitude means that for an observer in the ground, the ship will be moving very slowly - so it will be easy to track once found.
How visible it will be, though, depends on its albedo. If it is completely dark, it will only be noticed if it passes in front of something - another planet, or the Moon. If however it is as shiny as a solar panel, lots of people stargazing will see it as a shiny dot. It might be visible by day, with the right angles and equipment.
If it has the albedo of the regular asteroid, it may probably be found before it settles into orbit. In 2018, 41.9% of the asteroids approaching Earth with a closest distance smaller than Lunar Distance were detected before closest approach.
The huge majority of those were in the 1 to 20 meters size range, so a much large ship might be easier to spot oncoming.
2019 is not over yet, but we seem to be doing slightly better at detecting things before they arrive. I'd give your ship's odds of coming in unexpected at around 50%. If it arrives without being detected, then once it settles in orbit it might be a matter of hours to days until it is seen.
P.s.: at a distance from the sun close to the Earth, the solar system's escape speed is around 42 km/s. The flight path of a 100km/s ship prior to decelleration would be a very broad, open curve entering the system at one side and exiting it again at the other side.