First Bettas are not a schooling fish. You cannot put 2 female together otherwise one with try and dominate the other, hence the fin biting you have noticed. Second, the general rule of thumb with fish tanks are 1 inch of fish for every gallon of water. Since you have a 3 gallon tank and Betta's grow to be about 2 to 3 inchs you do not have the room for anything more than 1 fish in your tank. If wish to have your fish "interact" with other fish then you can 2 or 2 times a day hold a mirror up to the glass and allow her to protect and mark her territory.