Are they all the same breed or different breeds? Some breeds are just naturally gentler and more prone to being picked on. The two that are beating her up sound just mean, though. A hen can be dominant without being abusive. Most people wouldn't tolerate a rooster beating up their hens why would they tolerate (or certainly why would they choose) hens who do?
So what ever happened with your fighting girls?
I'm having the same problem, I have 5 and one is being picked on terribly. She runs by all the other chickens and hangs out in the coop a lot. I have started letting her out of the fenced area, most of the time with one other chicken. I'll see what happens with this strategy. Good luck with yours
I've got the same issue with my older hens that jumps on the younger ones, ..I was told to introduce a rooster. The first one was too aggressive I had to give him away after he punctured my legs, and my older hens got really scared of him at the point they would not come out of the coop anymore....Some friends of mine got 3 bantam roosters and now the peace came back in their coop.