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You are dealing with living animals. Anything can happen. Nobody can guarantee you absolute success or condemn you to absolute failure. Now that the legal disclaimer is out of the way......
You have an excellent chance of success doing that. Many of us have successfully done exactly that. I've done it with chicks I hatched from an incubator.
A few things that can help your odds of success. The younger the chicks are, the better your odds. The older they are, the less likely the hen is to bond with the chicks and the less likely the chicks are to imprint on the hen. The best chances are when the chicks are only one to three days old. Some people have had success with older chicks, but the chances of problems increase with the chicks' age.
Broodies cannot count. They cannot count the number of eggs, the number of chicks, or the number of days that have passed. People have had great success in giving a broody chicks after her only being broody a couple of days, but people have also reported failures doing that. Being broody is hormonal. It's probably better to let the broody be broody a few days to let those hormones take full effect. Some broodies have been known to stop being broody after a few days also. Those hormones raged but didn't quite tip the scales to fully committed to being broody. I don't know that you have to wait that long, but if you can, I'd suggest waiting at least five or six days. I think it will improve your odds of success. But the age of the chicks is more important. Get them and try it as young as you can.
The method I recommend is to wait until it is totally dark in the coop and slip the live chicks under the broody using as little light and commotion as possible. She will feel and hear those chicks under her all night and wake up with a new family. Although she can't count, I'd suggest removing any eggs, fake or real, that you have under her. That's just to give the chicks more room. It's not an absolute requirement.
Good luck and once again ,
