With all those pin feathers coming in on the back of her head, she is likely the survivor of some kind of attack. And she is not a chick. It's hard to determine her age based on the picture but she is fully feathered.
I would quarantine her for 30 days before you begin integrating her in with your other birds. Is your setup large enough to accommodate her?
Feed her a good quality complete feed such as a chick starter, All Flock or Flock Raiser type food if you are in the States. Something with 18-20% protein and NOT a layer feed with excess calcium. I would also offer her a tablespoon of tuna once a day for a week or so to make sure she's got good quality protein on board and one dish of plain clean water and one with poultry electrolytes in it. Then you can switch to tuna (or sardines) once or twice a week and just give her plain clean water.
You can put her in a wire dog crate for outdoor time well AWAY from your flock and down wind preferably. Tend to your flock first then to her. Always wash up and wear different clothes between working with her and then your own flock to prevent disease transmission.
After 30 days is up and if you see no signs of disease you can begin slowly integrating her into your flock. It will be very tough for her because she will be going in as a lone bird to two bonded ones so go very slowly using the "look don't touch method" for at least a week before letting them intermingle. Make sure there are multiple places with food and water, lots and lots of space with lots of things to do in the space including places to get out of the line of sight and places to jump up on to hang out and preen.
Good luck with her and thanks for keeping her and giving her a home.
