Our chicks were about 4 weeks old when their brooder light blew out and I couldn't find the other one to replace it. It was warm in the house by then except for about 4 days, but they did fine. Their brooder was wire dog crate that I put those inter-linking exercise mats around and on top of, so they were cozy. They are now 7 weeks old today and they are out in their new coop and run.
It really depends on how quickly they feather out, and on your house or outdoor temperatures. At 3 weeks, they should have temps close to 85 degrees, maybe even 80 depending on the chicks. So if you have those daily temps, you could just use the lamp at night... Really, watching the chicks will best tell you whether they're too cold - if the tend to pile (to keep warm) and cheep a LOT (as if distressed), then they're too cold.
cheers iv been terning there heat lamp off in the day when the suns out and they seem fine with that. i'll give them anuther week or so and see what the crack is then (i.e outside temp and feathers). what will happen if they dont get the propper heat i no they probs will die but how about othere things.