To be honest, it's not the size of the pots that is going to be the problem... nor IS it a problem! The pots are always going to stay the same size.. it's the plants that will grow. You're going to have a space problem anyway, trying to fit 3 autos in that size tent but there you are... smaller pots will NOT equal smaller plants unless you go to something like a solo cup! LOL! And autoflowers absolutely hate being transplanted so please don't do that.
What I would suggest you do is plan to do some serious LST (and you can LST as often and as much as you need to) ... and you'll probably have to do some defoliation as well... which will probably include choosing only the best bud sites to leave and trimming off a LOT of lower bud sites.
Good luck! You're going to have a jungle! Make sure, when they get into flower, that you keep the humidity low (30s-40s) so you don't run into fungus/bud rot issues...