You don't have to wait 21 days, but I agree it is better to wait a little while if you can. Some people have reported a broody accepting chicks after just a couple of days of going broody, but I think your odds of success increase if you can wait. If you have eggs in the incubator, you don't have a lot of control over the timing.
I don't know how many chicks you are talking about or how valuable they are to you. Sometimes people pay a lot for eggs and don't want to risk them any more than they have to.
Last summer I had a broody and also had eggs in the incubator. A snake was eating eggs out from under the broody so I wound up with a slightly drawn out incubator hatch. I took four chicks that had hatched and dried off and put them under the broody about 10:00 at night. She accepted them no problem. A couple of days later, after the rest had hatched and she had brought the four off the nest, I put her in a prepared enclosure. Then I mixed the four chicks she had accepted and the rest in a box, then dumped that box at her feet. She raised 15 chicks.
Probably the best way to go about it is to give them all to her at night. That is probably the best chance for success. But if you don't want to risk them all, there are other ways to go about it. They are living animals and I cannot tell you what will or will not work. They keep surprising me.
Good luck!
