Lost one hen (possible respiratory issue?); concerned about another...

Hello. I have strolled through previous posts but haven't found an exact similar issue, so here goes...

We had three hens one and half years old. We thought one hen was going broody as she stopped laying six weeks ago. A week ago we noticed she was no longer eating, sleeping on the ground and sitting most of the time. We pulled her away from the others five days ago. She appeared to be drinking but no eating and started having trouble breathing. Three days after she was gone.

One of our other hens seems to be eating well with a nice red comb. The other is eating a little but has not laid in ten days and I'm becoming concerned that she is heading down a similar path as our dearly departed hen. She has lost all of her tail feathers as I believe the hen who passed away did. The hen who passed away also lost a some off of the middle of her back but nowhere else. Due to the lack of eating I was unable to determine anything from her droppings (or lack of droppings).

Any thoughts or advice? Do respiratory issues present themselves with losing feathers and not eating? Did another illness lead to the respiratory distress of our departed hen? Is there a recommended antibiotic or other medicine I should be considering, just in case?

Thank you.

Tony