Each chicken is an individual. I don't think any of us can predict what that specific one will do, just go by what we have done and maybe you'll get similar results. I have never tried to give chicks to a hen that has set for a short time period. I don't think they can count since some will stay broody on a nest for a lot longer than 3 weeks. I have taken freshly hatched chicks, stuck them in the nest with the hen at 4:30 in the morning, and she accepted them. She had been setting on plastic eggs for about 3 weeks.
I think what I would do in your situation is put a few (three or four) eggs that you expect to hatch under her now, realizing they are at some risk. Only expose a few, not all of them. Let her go through the whole hatching thing, chicks peeping in the shell, pipping and zipping, and all that. I did not do that and it worked fine, but maybe this will help reset her hormones and expectations. Observe. If she accepts the first to hatch, you are OK. If she hurts it, it did not work. You'll have to decide for yourself if you put the other eggs back in the incubator and risk shrink-wrapping them and the others or just writing them off. I'd put them back in the incubator myself.
If she accepts the hatched chicks, I'd feel comfortable putting the others that hatch in the incubator under her at night. I don't know how she would be able to tell the difference between the ones she hatched and the new ones if they spend a few hours under her and they are there in the morning. Another advantage of chickens not being able to count.
If she had been broody longer, I'd agree with just putting some under her at night, but I don't have the experience with broodies that have just gone broody accepting chicks. Another possibility is to just put one or two hatched chicks under her at night and see if she accepts them, so you are risking only that one or two. If she accepts them, then try giving her the rest. Anything you do is going to have some risks. I think most of these have a pretty good chance of working, but I give no guarantees.
The way I did it was to put 4 chicks under her at 4:30 in the morning, risking only the 4. I left her alone until she brought those 4 off the nest, then put her in a tractor and her 4 in a box. I added the other 10 from the incubator to those in the box and dumped all of them into the tractor. She accepted all 14. When I took the initial 4 out of the incubator, I shrink-wrapped one hatching chick. It was still alive when I took the other 10 out of the incubator so I soaked it in a cup of warm water and picked the shell and membrane off. The down was still gunky. I stuck it back in the incubator until it had dried off, then just set it on the ground in front of the hen. She accepted it no problem. It took about a week for the gunk to wear off, but it thrived and is doing as well as any of the others. A big influence in why I did it this way was that I had a snake that ate the real hatching eggs out from under the broody and did not want to expose a lot of chicks to the snake. If the snake had not been around, I would have done it differently. Another example of different situations.
Your hen is different from mine. Your set-up is different from mine. I can't tell you what will work in your situation. Hopefully you can pick something from my experience that will help you. Good luck!
