In my oppinion they are to young to introduce them to an older flock. I would have pens close together so they can see eachother for a while before integrating them. I always wait until they are almost full grown before integrating them, by then they can better defend themselves. Good luck!

I waited until mine were about 3 months old and then just chucked them in with the other chickens. They sorted everything out after about a week :P

It might be best to have a transition period where you only leave them out with the flock during the day, and bring them back in for the night. That'll give the older chickens time to get used to the new additions, while also giving the chicks a break every day.

But you should be okay, despite the fact that the picking might seem bad at first.

I made an area in the coop with a hole just large enough for a smaller chicken or babies can squeeze through. It's made of regular wire so they can still see what's going on and if it's going to get too chilly I can put a heat lamp over top of them.

It's most useful for those randomly extra-stinky batches of babies that make the entire house reek and the husband give that special look that hints of stew.

My opinion is that 3 weeks is way too young to try to integrate into a flock with older birds.  They likely are not even feathered enough to put outside yet unless you live in a very warm area.  I agree that they need to be close to the same size as the older birds before attempting to integrate.  As someone else mentioned, I put them in pens side by side for a few weeks before integrating.  Then one day I take down the fence separating them and they integrate smoothly