Two things I'd try. First is make the 2 boys smell like the girl, and/or vice-versa. That's worked for me in the past, or take all three to the vet's office and let all three "pick up" the smell of the vet's office that is mixed in with many other animals. This seems to make a difference once they are all back home.
The other thing is to install a temporary screen door on the room that the girl is in. (Use the existing hinges on the door frame) They can see one another, smell one another, but can't fight or get hurt.
Good luck. Cats are difficult to figure out. I guess that's why we love them so.