I agree, you can put two females together as long as they have caves and leaves to hide in. But unless your trying to spawn them thats a no-no (no two girls with one boy or one girl with one boy unless your planning to breed them) Betta tend to Kill the females especially if she isint receptive to him. Try using a divider to keep the girls away from the boy. I've seen girls be with a boy for a day or two the next she's dead.
As betta can be placed with certain tank mates( platies, tetras etc..), female bettas are not reccomended.