First of all, welcome to BYC! No, you can't make her sit on those eggs for another 21 days. You will need to just incubate them or eat them. Are her current eggs fertile? When hens go broody, they don't eat or drink as much and only defecate once or twice a day. They put everything they have into hatching those eggs, even if they get super thin, dehydrated, sick, etc. It takes a toll on their little bodies. Once they finally get done with those 21 days they have to work hard to gain bak the weight they lost, and they work hard to care for those new chicks for the next few months. If you make her go through that whole incubation period again, she will get very sickly, dangerously thin, and she probably will stop sitting, leaving you in a difficult spot. You will either have to run out and get an incubator, (local ones usually aren't very good quality ones) or you will have to cull the poor young embryos. There is almost no way she will make it for 42 days of sitting, and still be able to properly care for the new chicks. If you had given them to her a week to a week and a half in, maybe, but now after the 3 weeks I think it is too risky. Sorry.