Rotate the bubs, take one of the older ones OUT and bring inside in the warmth in a box and wait until they feed the little one then rotate back. Otherwise u have too hand feed. Dont put more than two chicks under at a time and keep rotating and when a few more days older, they will probably feed all three. I have seen pairs rear 4 at a time. If smaller and with the bigger chicks in nest they will feed the one that begs the most so the older ones will be more aggressive for begging and get the food, if you do as I say and rotate, they will feed baby. Otherwise you will have to hand feed and if you DO have to hand feed, leave the small one with fosters and take an older one and feed IT.