You could get a male and two females, however that cage may get a little cramped. I at one point had 3 females in mine and there just wasn't enough room.

You want to make sure they are all of breeding size/age. Crested geckos are very prolific and will pretty much breed at any given time.

I keep my 29 gallon just like it is. You could turn it up on its side for more height if you wanted to. I just found it to be a pain trying to get the screen lid on and off with it being turned upright.