Our run is 32 square feet and the coop is about 10 more square feet. The birds are almost a year old. We feed them layer crumbles and they also get out in the yard and eat stuff they find around, but there's snow on the ground so not as much to find.
Does the 10sqft coop include the space the nests take up?
That space may be a bit tight for 3 birds,
tho it matches the oft cited but bare minimum 4/10 'rule of thumb'.
Is your run covered with a solid roof to protect from the weather,
or is ground covered in enough snow they won't go out in it?
Is your run an empty space or are there roosts and other thing that the birds can get up on or underneath to get away from each other and provide some 'enrichment' activity?
Crowding is likely the main issue...'Cabin Fever' is real,
maybe low protein depending on the percentage in the layer feed.
If they are not allowing her to eat, and that's why she's thin, or she may be ill, that weakness can bring on aggression.
You might try to take one or both of the bullies out for a day or two to give the picked on bird a break and let her get some food. Then add the bully birds back one at at time. Multiple feed and water stations might help too. Or even move the birds in and out every few hours to create a distraction that might break the bullying habit.
Tough situation, small space just a few birds, hard to separate in place, removing and adding birds back brings integration issue. If you could put a wire dog crate in the run to separate in place, that might help too.
Knowing your location and seeing pics of the coop and run might help us come up with other possible solutions.
