You could absolutely manage freezing temps with a 3-season bag by adding a fleece liner and hat, etc... I am now using a summer bag which isn't warm at all. When we went through the high Andes in the middle of winter (temps got down to single digits F at night) I added a fleece liner and two hats on my head. I also slept with my down vest on and draped one of my son's down jackets over my hips, which is where I was getting cold. It took a bit of trial and error, but I finally found a system that kept me warm and toasty at night.