You will be surprised how big of a difference three weeks makes. I thought I could do this with chicks two weeks apart, but when the second batch arrived the size difference was alarming and the small ones were too delicate and I was afraid they would get trampled. I thought I'd wait a few weeks till the little ones were stronger, but it has now been four weeks and I am still too scared by the size difference to put them together. I have the older chicks down in the coop in an enclosure and the younger chicks in the brooder still. I think I will wait a few more weeks and then put them all in with the big hens together so that there will be three different groups and really screw up the pecking order so not just one group is getting picked on.
That's just my idea - see how it works for you.