Sorry, I would not give Benadryl.
Both Fowl Pox and Infectious Bronchitis are viral - an Antihistamine is not going to help. A virus has to run it's course. Fowl Pox that is uncomplicated will run it's course in several weeks. Infectious Bronchitis, the same, but often it accompanies another...
Can you get some photos of the others?
Your still warm enough to have Mosquitoes, so Fowl Pox is a good possibility.
Have you noticed if they have any bubbles in the eyes, mucous, rattling breath, coughing, sneezing, lethargy?