they are in a run about 15' x 8' but they do have a good run in our garden (which is very large) most days. I have checked for mites, and cant see any but have treated with a natural powder just in case. I am wondering if they have eaten something now. Thanks for your reply
When I saw the pictures that was the first thing that came to mind, was they got a hold of something, because it's very odd that all three chickens have the same rash as the others. I would try putting something on the rash, I have read people using cortisone, perhaps a warm bath would help clear up the rash and then rub some cortisone on them.