10x10x15 isn't a cube and you shouldn't calculate your tank space as different areas, you can't keep fish in set areas without physical barriers. I'm assuming you're talking about a full sized aquarium here, give us a bit more info such as..
1. The length, width and height of the aquarium
2. What type of aquarium? Unplanted, lightly planted, heavily planted? Lots of hiding spots, bare tank?
Female Bettas can be kept together provided they're introduced at the same time, have been kept or raised with other females and there are enough hiding spots/line of sight is broken.