How many other chickens do you have?
For treatment get permethrin poultry dust, dust them down and rub it into their skin, all over them.
Get some Nutri-Drench too, it has lots of nutrition. Give them some directly to help prevent them from getting lethargic. Until you get the Nutri-Drench give them some gatorade.
Put some garlic cloves and ACV in their water too. 1 garlic clove per gallon and 1 tablespoon of ACV per gallon.
You need to clean out their coop and spray it down with permethrin, here's a link to dilution.->
https://www.backyardchickens.com/threads/predator-mite-question.1504940/post-25218638