It very much seems as if she has a serious respiratory illness. I would give them back to the person that gave them to you and let them deal with it.
The best thing for the flock would be to cull all three birds as respiratory disease can lie dormant in the body and flare up and become...
Yes, it's something to be concerned with.
Do you know the person who gave you this bird?
How old is the bird?
Has she laid any eggs for you yet?
Have you checked her crop function to see if it's empty in the morning and full at night?
What do her poops look like? Are they watery opaque with...