Awww, I'm sorry for the loss of your lil' ones. Where are you in Florida? I'm in Palm Beach County, the air here is like broth. Perhaps you should check your chicks & their set-up to be sure it's not something else besides the heat that's killing them. Three-week-old chicks are vulernable to a lot of things, if all you do is lower their coop temp you might be missing something else. I have chicks & chickens outside here and they seem to be able to survive the heat on their own. They have airy coops & pens and a lot of shady places to rest in & pant. They also like to dig down in the dirt and scoop out big craters to roll in, it must feel cooler there. I hope you figure out what's bothering them, and how to remedy it soon.