I potted 3 seedlings 3 months ago (1 Bridgesii, 1 Peruvianus, 1 Pachanoi) of about 4 cm each.

Only the Bridgesii has grown. The other two are still the exact same size, so I was beginning to suspect they might be dead. They're still green though, so I re-potted them, and saw they still have roots as well.

Bridgesii grow faster than the other 2 in my experience, so that doesn't sound to unusual, especially for repotted seedlings.

Do you give them fertiliser and full sunlight?

I've given them fertiliser a few times, with 3 weeks in between.

They first started out in full sunlight. However, June turned out to be a very hot month, and the Pachanoi and Peruvianus had actually shrunk a bit, so I put them in a more shady spot 2 months ago, fearing the sun would dry them out. Still haven't grown since, although they definitely don't look dead. Then again, I'm new at all this and I wouldn't know how to tell a dead cactus from a live one.

dont feed or water much-if at all-, or give full sun until they are growing!
rot and sun burn are the enemies

The Peruvianus has finally started growing. Now hopefully the Pachanoi will follow

When it stops breathing.