I have a K3 bag on a K2 frame. Works great. Inwas perfectly happy with the K2 bag I had before. But, I really liked the dedicated hydration pocket in the K3 bag. So, I swapped it out.
Yes, the K3 will fit. Want to make a worthwhile upgrade? Get the K3 belt. Significantly more comfort, wider webbing, better padding. Literally made my pack feel 10 lbs lighter. Make sure you get the correct belt, the one made for the K2 frame.