But, at three weeks, they likely won't need a 250Watt type infrared bulb. Those draw so much power and at this point, the weather most places is getting pretty warm and at 3 weeks, they would do fine with a smaller wattage, red bulb.
I use these, found at all the Big Box stores like Home Depot, Lowes, etc. The come in 100 watt and other wattages.
As always, make sure your chicks have a cool area to move to. Chicks self regulate, by moving in or away from the hot spot. To do this they need cooler areas of the brooder.