The rule of thumb is start off somewhere between 90 and 95 degrees F the first week then drop it 5 degrees a week until they are feathered out. After they feather out, it does not matter. That's usually around 5 weeks or before.
0 to 7 days = 90 to 95 degrees
8 to 14 days = 85 to 90
15 to 21 days = 80 to 85
22 to 28 days = 75 to 80
and so on.
So if yours are 3 weeks old, this means they are over 21 days and should be switching to 75 to 80 degrees F
I don't follow this at all. I'd go bonkers trying to. I keep one area in the brooder a safe temperature and let the rest of the brooder cool off as it will. I have a fairly large well ventilated brooder in the coop, so it will cool off to whatever ambient is. There have been nights where that was in the 40's. By providing one area that is safely warm and allowing the rest to cool off, I don't have to worry about keeping the entire brooder a perfect temperature. I let them find their zone of comfort. What actually happens is that they play all over the brooder and go back to the heat to warm up when they need to. They do usually sleep fairly close to the heat source.