If you've had them for 3 months, whatever it is is not the hatchery's fault.
They sound to me like they have been poisoned. Did you just open a new bag of feed? Have they had access to fertilizer, insecticide, rodenticide, fly granules, moldy feed (or moldy anything)?
I always feed medicated feed to hatchlings that have been shipped from a hatchery. Shipping is stressful and lowers their resistance. Also, they have no mother taking care of them to share her immunity.
At 3 months, though, I wouldn't still be feeding medicated feed. Depending upon what their stools looked like I might still be putting a spoonful of yogurt into their waterer once a day. Also, if they are not on feed made for ducks or gamebirds, I would be adding niacin to the water until they were outside and foraging. Chicken feed doesn't contain enough niacin for anything but chickens.