What to do for ill chick

I recently picked up some chicks and two older but not full grown hens from a local neighbor. The two bigger ones were in a coop. When we went to look at them there was one laying on the ground very lethargic. He took it out and put it in another area and told me the others were probably not letting it eat or drink. I am new to chickens so it seemed like a reasonable thing.

Within a few days one of the older hens had a goopy eye and was making a sound that sounded like a sneeze and was lethargic for a few days. She is doing better now. No goopy eye anymore and less sneezing.

However now one of the little chicks has sunken eyes that are crusty and it is extremely lethargic. How do I know if this is bird flu or not? What can (if anything) can I do for the chick?

They are separated from my coop and flock and I have been washing hands when going between their spaces to try and make sure the original flock doesn't get sick.