You need to nip this in the bud straight away or they're capable of setting on her and doing a lot more damage than just feathers. Can you observe for ten minutes or so and work out just what the pecking order is? If so, take out the top three and house them somewhere else for a week. If the original group settles down and there's no more pecking after a couple of days try reintroducing no. 3 in the pecking order from the group of 3. She should go down to the bottom of the new order. If this works continue for a couple more days before reintroducing no. 2 and finally no. 1, who should now be at the bottom of the whole pecking order. Other things to consider is how much space they have, is there something like a couple of haybales the shyer ones can escape behind, do you have some cabbage leaves or similar to hang up for them to direct pecking behaviour at and is the housing free of red mite - the irritation from mites biting can cause the hens to become very aggressive.