I think the problem is that Betta fish are so different to mainstream community fish in the way they are kept. In a 6gal community tank you could put roughly 6 x 1" fish in there. But Bettas need to set up territories - with females you should keep either one fish on its own (or in a community tank) or at least 3 females together.
Yes, sizewise you can put 3 females in a 12" cube tank but I'm not sure how they would go about setting their territory in such a small space. There might be a few battles going on! Having never tried it myself, I've no idea if it would work.
Maybe someone else on here might have some experience of having tried and tested it and can let you know if it works out or not? I suppose also it depends on the temperament of the fish as they are all different. Some are more feisty than others.
Athena