You can keep females together year round as long as they are in the same size range and don't act aggressive towards one another. Some people even house trios (one male two females) year round, but I would think keeping the males and females separated when it isn't the breeding season would be best for cooling purposes (I personally have never bred so that would be something to talk to people with experience in that category).
As far as housing goes. Some people house three in a 29 gallon enclosure. Some keep individuals in 20 gallon tanks and others keep individuals in vertical 10 gallon tanks. From everything I've read, seen, and a little common sense, I'd say try and give each of your geckos plenty of space. Doesn't it seem cruel and selfish to try and cram a bunch of geckos into smaller enclosures instead of giving fewer geckos better living conditions? I understand that many people say they do fine in smaller tanks, but I'm sure they'd prefer more space if they could have it. I plan on keeping a pair of females in one of the 23h x15.5 x15.5 Sandfire Dragon Ranch enclosures.