help 3 weekold six black copper marans had 6 down to 3

[getaclue]

Call your state vet. Many states offer no cost, or low cost necropsies. At least learn what you are dealing with. Not only should they not be near your flock, but I personally would be freshly bathed, wear fresh clothing, make sure my shoes are not the ones I wore around the chicks, and above all, make sure your hands are clean. I get the impression this might be more than a simple case of bronchitis, or pneumonia.

 

[Eggcessive]

It looks like mycoplasma (MG,) but you can try to either get testing, or cull and send 1 or 2 into your state vet or poultry lab to find out. Chicks can get MG from the flock they came from, through the hatching eggs of an infected parent, and they can also get it from the new flock they have entered if there is a carrier. Here are some good articles to read that might answer questions:
https://poultrykeeper.com/respiratory-problems/mycoplasma/
https://extension.umaine.edu/livestock/poultry/mycoplasma-gallisepticum-faq/
http://www.hobbyfarms.com/mycoplasmosis-in-chickens-2/

Here is a list of state vets and poultry labs:
http://www.metzerfarms.com/PoultryLabs.cfm