I'm guessing 42 x 28 is inches?
Some people might have different views but imo that's nowhere near enough space. By three weeks old, they need room to run around and start to fly. If they're indoors they shouldn't need much, if any, additional heat by that age either - especially when there's a dozen of them to snuggle up and keep each other warm.
For comparison, I have a corner of a room sectioned off that's about 80 x 60 inches. The five chicks in there right now are 23 days old and that's only enough space because they're spending lots of time outdoors. I couldn't take them out yesterday for various reasons and they were really bored and stressed out.
Do you already have a larger space available, or a plan for where you're going to put them next?