Where did you get the 3 week old chicks from? Most likely all of the chicks are already exposed, so probably no reason to separate her. She needs to be with the flock for company, unless she is so sick she gets picked on.
Do you have adult chickens? Chicks get respiratory diseases from a carrier, but sometimes they can get MG through the hatching egg from the parents. Chicks can also get brooder pneumonia (aspergillosis) from mold spores in the brooder usually from water spills in bedding. Make sure that they have plenty of overhead ventilation, and dry bedding. Raise feeders to shoulder height.
Mixing day old chicks with these older chicks may not be a good idea, due to being exposed to a possible respiratory infection, getting coccidiosis feom the older ones, or getting picked on due to size difference.
If you suspect mycoplasma (MG,) it can be treated with Tylan 50 injectable given orally 1/4 ml per pound 3 times daily for 3-5 days. MG can be mild or cause bubbles in eyes, sneezing, and lethargy. If she has infectious bronchitis, then that is a disease that lasts about a month, and most chicks may get it. Symptoms include sneezing often.
