Our chicks, only nine days apart in age, could not have gone together immediately, we didn't put them together until they moved to a new location for both groups, at two and three weeks of age.
Mary
Three weeks is a huge age and size difference, you will need two brooders. And I don't see how that heating pad will work for them; are you making a chick brooder using it? Just having them on it isn't enough, unless your ambient temps are 90F all night too.
We had two brooders going in our...