So we have a space-saving species that build an space ark to survive a cataclysmic event on their homeworld and save their species. They have successfully terraformed the new target planet but now have a 3 space arks to maintain. Assuming they don't want to colonize another planet what are potential uses for these spacecraft?
Strip it for parts
Your ship is going to be large and will be made up of many processed pieces: support beams, shielding panels, appliances, computers, plumbing, conduits, electrical and data cables, generators/engines/reactors, etc.
Strip it for parts. Even if you have to dismantle it in space and then shuttle the parts to the surface, doing so is probably faster than all of the steps necessary to build those things on the ground: locating raw materials, digging mines, building processing/refining plants to process the raw materials (without the processed materials to do so)...
So repurpose anything and everything you can from the arks to start your colony on the surface. Shields become walls and roofs. Appliances and computers are reused as-is. Generators too.
These won't last forever. But they should last long enough for your colony to build out the infrastructure necessary to create their own.
Even the materials you can't reuse as-is can be recycled into the raw materials for other things. Melting down the steel to make new materials is going to be faster than mining, refining, and then making new materials from local resources.
Maybe some parts of the ship are left in orbit. Why not keep some parts of the original ship up there and build it out as a science station? Or as a weather observatory? Or to build/deploy satellites? Explore other planets in the system?
Maybe the stripped-down arks become mining vessels, sent out to recover asteroids because that's more environmentally safe than surface mining on your new colony world.
Probably, you're going to end up with a mix of all of the above. Much of the ships are brought to the surface to serve as first-generation dwellings while everyone works to start food production and the critical industries necessary to become self-sufficient. But some parts will be just melted down / torn apart / etc. to make raw materials. And the left-over skeletons become ships capable of exploring the new solar system and/or mining non-habitable worlds/asteroids to supply the new colony.
Think of it like this: Why worry with the additional weight of packing flat-pack shelters and furniture and appliances when the ship will be full of them? Much easier to keep using what you have than to build new.
Colony ship, again x 3!
The thing about getting your homeworld cataclysmed: it induces a certain fear that you could get cataclysmed again. And fool me twice, shame on me. Putting all the eggs in one basket is not the best strategy.
And here are these low mileage arks, good to go and with plenty of tread left on their space tires. Also: their terraformer teams learned a ton doing up their New Target Planet, but now just sit around talking about the glory days, collecting their pensions. Getting that crew off world would be good. And there are a bunch of young folks who are keen to make a new world of their own, with a name more hip than New Target Planet.
Each of the 3 arks is sent off to three new potential homeworlds, with the goal of replicating their initial success. There is a bit of a competitive fraternal spirit between these three. But also a lot of love and as opposed to the first time when all there was back home were radioactive zombies, this time each of the 3 can support each other, and also be supported by their folks on NTP.
I think CaM is on the right track with the infrastructure bit. Why waste 3 perfectly good, ready made space station/mining platforms? As they are arks, even with the space-saving nature of the people (or am I interpreting that wrong?), they would still be fairly large which could be good for an orbital hub of some sort.
Obviously the tech base is there, so regular space travel could be a thing (or possibly not, is civilian space flight a factor?). Turning one into a mining hub near the whatever counts as the main resource area and another into a transit hub orbiting the planet could work well. Depending on the drive emissions and size of the resource haulers, you might not want them entering your atmosphere.
If your people haven't yet discovered the virtue of pacifism or, conversely, have discovered that the universe will do it's very best to kill them, then perhaps a large, mobile, military command and control/emergency management facility is warranted as well.
Why not keep it as a monument or even make a museum out of it? The future generations will surely appreciate that. It will stand for a new beginning. A first step. But it will also remind the people that they had to leave their old planet and whatever mistakes led to that, they have to be better.
Live in them
Your colonists have just spent generations traveling to, and then terraforming this new planet. They've lived on the arks all that time, and the arks don't suddenly become uninhabitable just because you no longer care to use the engines. Many(all?) of your colonists have no idea what living on a planet is like anyway. Some would prefer to stay in space, they can always visit the planet, after all.
There's lots of things that can be done in microgravity that are more expensive/difficult to impossible in a planet's gravitational field. Growing many types of crystals, making "perfect" ball bearings, etcetera. Plus, getting rid of waste gasses and heat is trivial.
Burn victims, cardio-pulmonary issues and the like will be easier to live with and provide lower risk settings for recovery in weightlessness.
Max Security Prison
It's a lot harder to escape and "blend in" when you have to leave orbit in a small shuttle. If the prisoners riot, just start lowering the oxygen level.
High Risk Research
Say you want to play with space warps, nanotech or genetics. Here's an isolated place to do that with lower risk of it endangering the population.
So you’re all done terraforming and probably feel like having a bit of a symbolic gesture to celebrate you’re done and not going anywhere else in a hurry - so how about some really good fireworks to celebrate. Feels like option,
- Bring some/all of them in on a steep (enough) reentry (after all we don’t want another cataclysm on our hands)
- Presumably there’s also a sun. Oh and if you haven’t met your new inter-planetary neighbours yet - maybe keep a spare.
They have successfully terraformed the new target planet but now have a 3 space arks to maintain.
Actually they have successfully terraformed the planet BECAUSE they scrapped the ships.
Just like Hernan Cortes in Veracruz, at the beginning of his conquest. Why did he do it? Because retreat is easy if you have the option. Because having an exit strategy means being able to not really commit to the project. Because after generations of comfortable living in the controlled environment of the ship it is scary to go down to an unknown, hostile world and work hard there to make it habitable.
But what is needed is exactly acting with determination, in spite of fears.
This is an extremely dangerous action for a whole population, aye. But completely needed.
How you do it is up to your story. The leader(s) could just issue orders if they were to have te authority Cortez had (he had to hang and lash to get his way) or simulate an incident or (even better) a terrorist attack. Why the last is better? because it gives the population the threat they need to gather around the leader(s) and be guided down to the planet towards a bright future....