New Chickens Fighting!

I recently introduced 3 chickens to my 2 girls. They seem to have established a pecking order with one of the newbies at the top and the other 2 at the bottom. The two at the bottom seem to be being bullied, my original two jump on their back and pull out their feathers, one of them has no feathers around her neck (although she was missing a few when we got her). We've had them about 3/4 weeks - is this normal, is it ok behaviour or is there anything we can do, its not very nice to watch!