Was it mustard coloured when it was born? In which case the birth fluids may have been stained with meconium and the lamb very stressed by a lengthy birth. I'd take its temperature and, if low, either remove all three to the house or, if you can bottle feed or hope to later adopt it onto a freshly lambed ewe with a single, take it on its own to the house, put it into a jacket made from a rectangle of old sweatshirt or similar and put it in a well bedded large cardboard box in your warmest room. Get more colostrum into it asap, as its brown fat will start to be exhausted after 6 hours. If it can't hold its head up you'll have to stomach tube it - if you're not confident doing this contact your vet or an experienced shepherd if you know one. You have a very small time window to get it going. Did the ewes have a booster vaccination 4-6 weeks before lambing?