I don't have much experience but will try and help as best I can.
You will have to separate it frome the others somehow, at three weeks you should be able to keep it at a good temp even with a regular bulb if need be, and it looks like you are in CA so you should not have to worry too much...
I have a Rhode Island Red chick about 3 weeks old. My father-in-law got 50 from a hatchery. About 6 days ago I noticed this eye problem. I had left her with the babies in the brooder, as she seemed to be doing okay, I suspected maybe a tumor, which unfortunately I can't do much about except keep...