We had this issue when we introduced our RIR to our two BAs. They're only two months apart, but the older two still had to assert their dominance over Baby. We set up an area in the run for Baby to escape to and get her own food and water. Since she was still young, we set a rubbermaid tote on it's side at an angle to the side of the run giving her just enough space to fit through. After a week or so of that, she gained some courage and the other two had gotten used to her enough.
There's still some rivalry, so we have a feeder in the run and a feeder in the coop. That's solved any squabbles!
There's still some rivalry, so we have a feeder in the run and a feeder in the coop. That's solved any squabbles!