You sound like you are on track. What you do need to do is to get up and walk several times a day-go a little farther each time you get up, even if it is only one or two steps more. You should also be sitting up for a period of time each day. Not long each time, just 10-15 minutes. You do need to keep up with the walking as it increases the blood flow to your back and that will help with the healing.
Come check out the back board here. It has alot of good hints for after your surgery.
I started with the same 3 level fusion. Does your doctor have you in a brace?