I'd agree with Ch4. In my limited experience with a betta sorority, you want them 'overcrowded', similar to keeping African cichlids, so that each of them are protected from over bullying by having lots of targets for bullying from the 'alpha'-females.
3 probably will be less successful than say, 7. I had 7 in a 15 gallon, and they worked out their hierarchy fairly quickly. There was chasing, but nothing more than that. Lots and lots of flaring gill covers.
My experience was with related females, and I've heard that is easier than randomly sourced females. My females, I believe, all came from the same tank at the breeder's place before coming to my tank. If these are all unrelated females, you may have more trouble.