My sorority has 3 females now. It started with 6 females in July. One died mysteriously in about August and then about 3-4 weeks ago, two came down with dropsy. After they had passed (in the hospital tank, not the 10 gal) I treated the entire tank with Maracyn 2, as recommended by our LFS . All is well, now.
I spent some time seriously considering whether or not to bring the number of females back up to 5 or 6. The three I had were not fighting at all. NOT ONE BIT. They were actually more peaceful than they had been when I had 5 in there.
SO we decided to try feeder guppies in there with them. We were thinking:
1. Smaller, faster, different movement and look
2. less individual than the bettas, as they children were not taking to the deaths very well.
3. "FEEDER" fish, meant we could discuss with the kids that these fish were intended to become food for other fish, so if they were eaten by our Betas, it wasn't too different from what they were headed to originally. Call me harsh, but my eldest understands that animals eat animals and if she could think of it that way, she could handle any deaths easier.
4. at 4 for a buck, I could afford to throw away $1 or so to try out our female bettas with smaller fish.
two weeks later.... three female bettas and three hapily growing male feeder guppies are swimming in the tank. They've seriously doubled in size since we bought them. The bettas took two days to adjust to the little fishies, but now they all leave each other alone. Well, except the guppies who chase each other. LOL.
I wouldn't have tried it with fancy guppies and didn't want to shell out $15 for a school of tetras unless I knew they'd all work. And dag nabbit, we've all come to REALLY like the guppies. A lot.
So, I guess what my story says is that 3 females CAN work, and bettas plus guppies CAN work, but you have to be okay with the potential casualties if they don't.