Three is pushing it in such a small space. You can try, after the new female has been properly quarantined of course, but keep an extra cage on hand so you are prepared to split them up if necessary.
I would say its possible...but so is 10 in a 20 gal. The rule of thumb is 10g floor space per gecko. You would need another cage/tank for quarantine anyways. Get a 10/15 gal for the male and just keep your females together (after quarantine of course). Also if they are still juvies then that is fine for a while...but adults I would say thats too cramped
I would say no.. but I believe in having a 20 gallon for 1 leo. It's best to recreate your leopard geckos habitat, meaning decent space, plants for shade, my leos looove big rocks in their tank under their baskiing light.
a 30 - 50 gallon would be good to 3-4 leos if its a longer tanks - or you can do wat i did, flip the tanks on their side - 20 gallons have for height than floor space. build a lid that will fight tight on the aquarium. The leos have sooo much more floor space and i keep 3 in my 20 gallon breeder tanks and my leos are fine.