You only need one day to experience Rila Monastery. I see no problem with three days. For the Rila Monastery leave in the morning and come back at night. The trip takes no more than two hours each way. I would only be worried if it is snowing with 2'+ on the ground...in that case stay in Sofia. The only problem with snow is that it takes about a day to clean the secondary roads if it snows continuously. I've been there two times but that was more than 10 years ago.