Edited by tinar - 2/28/16 at 6:56pm
Sick 3 month old chicks:(
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DE won't get rid of a mite infestation. Use some poultry dust or Adams flea/tick pet spray. You must also retreat both the birds and their coop every 7 - 10 days to break the mite hatch cycle. A spray, like Sevin or Insectrin is easier to use in the coop, easier to get it into all the nooks and crannies where the bugs like to hide. Insectrin is good because it has residual effect.
These birds sound like they have some kind of respiratory disease. If they are sharing a fence line with your other flock those birds are now also exposed as these things spread so very easily. Antibiotics can be given to help prevent secondary infections, or to treat if what they have is bacterial, but without having some testing done you won't know what they have. There are quite a few different diseases and many of the symptoms mimic each other. Even if/when they recover they will still be carriers of whatever this is they have. How long they are carriers also depends on what this is. Try to find out where to get a necropsy done by a state poultry pathology lab so in case you loose another one you can get it tested to find out what you are dealing with. Or if you have access to an avian vet take a live one in to have some tests done. Getting a diagnosis in enormously helpful.
Unfortunately this is a good example of why it's not a good idea to buy birds from a poultry auction.