Actually, at 3 weeks post op you shouldn't be doing ANY core strengthening exercises. Ask your surgeon, but at 6 weeks they may allow you to do some very mild core engagement exercises, but not full blown core strengthening until around 6 months post op. You don't want to risk your fusion by not following your surgeon's orders.
The best thing you can do right now is walk, walk, walk. This helps keep the blood flowing and helps you heal faster, as well as giving you some cardio exercise.
I went to PT 6 weeks post op from an L4/L5 fusion, and my PT had me do a lot of leg/arm strengthening exercises with a stepper. Also, we did light engagement of the core muscles centered on pilates principles.