Greetings and welcome to BYC. I would NOT add a new baby chick in with 3 week olds without a long period of constant supervision to ensure the new chick's safety. There would be a HUGE size difference in that by 3 weeks, most chicks are capable of some flight and would out weigh the babies by a huge amount. If you could split the brooder into two areas, each having access to heat/cool and food/water, that would be best. Good luck!
I have been thinking about ways to do the same thing. I ordered some day old chicks and none of the bantams survived shipping. We have happy, healthy standard chicks that we're thankful are doing great! But we were really excited about having a bantam or two. You're not going to be able to just add new chicks, though. Even at 3 weeks, they've associated one another with the flock and will be territorial to outsiders. They would pick on the new one(s) and not let them eat or drink what they consider to be "their" food and water. I've been thinking about setting up an entirely separate brooder and putting it next to the other chicks. That way they could see and smell one another without directly interacting and then slowly trying to integrate them once they're old enough and used to one another. Under very closely supervised conditions over several weeks, of course. Don't know if that would work, but it would be easier than raising them and waiting for a hen to go broody before adding to the flock!
FYI chicks double in size every week when they're newly hatched, so 3 week old chicks would be 3 times larger than a day old chick!
