I would put her in a the run area, like in a wire dog crate. you can put a board or something on top so the other two don't poop on her head if they get on top of it.
Dog kennel used as pen to break a broody hen.
Oh my! She's quite the little pecker! The space is rather small so it would be hard to add perches and obstacles and extra feeding stations but that is one thing I would advise. So the other two have room for escape.
I would try physically separating the bully in a cage with-in the coop so she...