At three weeks, chicks do okay in freezing temps during the day while they're consuming calories. But they will need reliable heat at night. Unfortunately, heat plates do not provide reliable heat because the chicks don't make direct contact with the heat plate and it really doesn't put out enough radiant heat to combat freezing temps.
Heating pads do, however, provide the kind of heat that chicks require in freezing temps since they snuggle under it as they would a broody hen.
It would be best to just keep them inside for two more weeks until they feather out completely.
If they were 4 weeks old and fully acclimated to outside temps I'd be more comfortable with it. At 3 weeks, they probably still need some heat with those overnight temperatures. Main issue would be some heat plates specify that they should be used in 50F (10C) or higher for optimal performance, so if the temperature is lower than that it may affect performance of the plate. But given that they're 3 weeks old, if the plate is even working halfway it still should be enough heat for them provided the plate is somewhere draft free.