Make sure there is enough room for all of them. Try the search button at top of page, I've been trying to look for some answers, still looking. Are they picking to the point of feather loss and bleeding, or just pushing one another around?
So...black oil sunflower seeds will stop them picking and eating feathers?
My #1 hen has recently started doing this to the other two, and they always eat the feathers. They're getting commercial feed, oystershell and scratch grains, some kitchen scraps (some rice/pasta and veggie scraps) and go out for forage in the yard (they haven't gotten out as much recently due to the weather so I've been dumping weeds and grass in for them). They've got access to 16 sf indoors and 24sf outside (perches in and out), so even when they're in there should be enough room for the 3 (GL wyandotte, NHR/Ameraucauna, welsummer). The NHR/Ameracauna has been picked bald around her vent, but I'm putting antipick on it. #1 hen (wyandotte) will reach out and yank a beakful out of the welsummer (#3) just at random, then the bigger two will eat the feathers. #1 and #2 are laying since last year; #3 should start laying next month.