Really, I would not worry too much about the heat. I would worry more about them not getting fresh air and sunshine. By three weeks, they are pretty tough, and as long as they can get out of the wind, and are dry, they should be warm unless you are having day time temperatures below 30 degrees.
If your chicks start peeping madly, a high pitched continuous peep, not just the general constant peeping, that is a sign that they are too cold. Chicks do not have to be kept at a constant warm temperature, but rather do better with a cool place and a warm place.
Mrs K