You can try giving them to the hen with the existing brood, She may accept them, but It would probably be safer to give them to the broody who has yet to hatch. Hens can detect the slight differences in size and development in chicks even a few days apart, and for some, this can trigger the instinct to reject the younger chicks.The fact that she only has three chicks in her brood, may also cause her to attack the younger chicks as she will be aware that their has been a sudden increase in chick numbers, and this may arouse her suspicions.
Other factors such as the colour of chicks can also cause a hen to reject. if the original brood of three are all the same or of similar colours,any new chicks introduced of a completely different colour are likely to be rejected.
Why not try the chicks with Madge first?,If she is a docile, ' Mumsy' hen, you may not encounter any problems with the introduction.Try placing the three newbies under her at night, and be there in the morning as she leads them out, and watch her carefully. Some hens will appear to accept new chicks initially, only to turn on them a few hours later. If the introduction fails, save them for the hen due to hatch, you shouldnt have any problems there. Just be sure to allow the hen to have all of her own brood hatched, dried and ready to move before you place the fosters under her, say on the first night of the hatch day.This is because the incubator chicks being older and stronger, will be ready to move away from the nest area immediately, and they may draw the hen after them before her own brood have finished hatching properly or have found their feet yet.