ah yes...this is female aggression then. Females in general cant bite their own fins and they certainly cannot get to their anal fin.
Female bettas living in a school together will establish a pecking order and this is important for the territory each girl will have and how much say she has over the others. Occasionally tempers flare or the pecking order is challenged whih can result in some torn fins.
If they have lived together for a while with no aggression signs I dont recommend you take her out just yet, however if you see her becoming skittish, hiding a lot or becoming more and more ragged then we can assume the other two girls have teamed up together to gang up on her.
If this is the case your options are to either separate them all into their own tanks/behind dividers or possibly establish a slightly larger school of five or more. A large school's pecking order is more dynamic and you are less likely to have scenarios where one girl gets singled out.