Hi.
Sorry you are losing your chickens...
What can you say about your chickens?
What do you feed them...?
Do you use any chemical product they could have been in contact with?
Do they freerange? And if so : do they have access to any toxic plant?
(...Do you have a cell phone tower, wind turbine, or a solar farm near your area...?)
If these chickens were mines...
...I would carefully rub (a lot of) food grade Diatomaceous Earth on them, just in case they have lice and/or mites...
And, of course, spray it in their coop, run, and sand bath...!
And in order to get rid of worms and whatever parasites they could be infected with, I would also :
• 1 - add 1 tablespoon of Apple Cider Vinegar per liter of water (in a PLASTIC waterer; never put Vinegar in a metal one!!) - during 24 to 48 hours;
• 2 - mix food grade Diatomaceous Earth in their food - during 14 days AT LEAST.
(...Still : I hope for you you will get better advices!)
If these chickens are prolific layers, then at least the 3-years-old ones could "just" be dying from "old age".