Apologies that no-one replied sooner. I don’t have a definitive answer so can only pass on my own thoughts.
KnittyKnat. If there are 3 in a box and one is a larger youngster, maybe it was trying to gatecrash on another family and got chucked out (although litter mates can vary in size, with some growing faster than others). Maybe it settled down outside hoping to get back in later(?) It seems odd, if it is a Mother that she would be hibernating in the box with her youngsters still around, that close, especially if she had still been feeding one or both of them. Although it’s possible an adult hog hibernated there and the youngster (either related or not) sneaked in, but is/are not being fed. Mother hogs would normally eventually disappear to hibernate and leave the youngster(s). They will sometimes get a bit cross with youngsters if they follow them around too much after they are weaned. Likewise youngsters tend to tolerate other youngsters sharing, but tend to prefer not to share when they get a bit older – so one youngster could chuck another youngster out (or make it difficult for it). But, yes, the youngsters might welcome having a home each. So, another hog box (or two) might help, if you have the space.
Corporal Jones. Welcome to the forum. I’m not sure why a hog would want to sleep outside the hog house during the day (even if the house was already occupied by another hog who had evicted it), especially at this time of year. I would be a bit worried that the hog might be unwell and wonder whether it would be worth having a chat with your nearest hog carer/rehabilitator and take their advice. You can get contact details of your nearest ones by ringing 01584 890801.
Good luck. Hope ALL the hogs do well.