They were nine weeks old and fully feathered. All three dead ones were crowded into the same nesting box. I let them outside for the first time yesterday afternoon. They seemed very happy scratching and dusting in the sun. I put them back in for the night. I think they must have gotten fatally chilled. I must have underestimated how vulnerable they still are. The really bad news is that all three were Black Copper Marans pullets from Bev. The cockerels were fine, the mongrels I hatched at the same time for a neighbor were all fine. I only lost the best. Bev sent me fifteen eggs. I dropped one. Eleven of the remaining fourteen hatched. One chick died. Now this, and I'm down to seven, at least three of which are cockerels. Don't count your chickens until they're hatched, feathered, grown and laying. I'm kicking myself.