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Thanks @eJett and @CSULBMammalLab for alerting me to great question from @zoecology! North American porcupines have a huge geographic range so the answer may depend somewhat on location. Not sure where you saw these three in the tree. Here are a few thoughts about #porcupines: https://twitter.com/CSULBMammalLab/status/1328116991039660032
First, here in northeastern US, porkies will often sit up in the branches of a den tree; multiple porkies in a tree may be indicative of the group who are sharing the den.
Second, we actually know little about group denning behavior. Does it happen? You bet. Who dens together? TBD (we are working on this question in my lab).
Third, what is "winter"? A period of cold and a ground covered in snow? A period with limited food availability? Both? Again in the northeastern US, porkies are known to greatly retract their home ranges in winter. But my experience says that they reduce home range...
...before deep snow, likely in response to limited food supply. So it may appear that you see multiple porkies in a tree "outside" of winter because there is no snow, whereas porkies may be exhibiting "winter" behavior in response to limited food.
I'd be interested to know at what time(s) of year you have observed them in groups in a tree? Might also be fun to hear from @erin_sira who studies "social" behavior in asocial species...
