Having female bettas together is a process and is difficult to maintain the traquility so keep in mind the necessities of the fish as well as some rules to get your bettas to get along. This Thread
has information that will help you decide if having a "sorority" is right for you. This type of fish combination has its ups and downs and will require a lot of your attention to keep them healthy.
There is also betta community tanks, there are fish that bettas get along with because they don't have the long flow-y fins that make the bettas angry or small enough that the bettas don't see them as a threat. This Thread
shows betta compatibility and how much of each fish and size of tank that is recommended to keep all the inhabitants happy.