3 weeks post-op: should I massage my incision to soften it?

If your doctor says no then do not do it. I typically recommend massage decision because I believe this will slightly accelerate the softening of the incision. But I did not operate are you. Follow your surgeon's advice.

Any wound is best protected from external trauma. Massaging incision releases histamine which will contribute to swelling.

Each surgeon has his own post-op recovery protocol for incisions. I prefer to keep them taped as long as patients tolerate it. At least 6 weeks.

It is almost always best to do what your surgeon recommends. We all have slightly differing approaches to scar management. The scientific evidence suggests that silicone will improve scar outcomes. I usually recommend the use of a silicone gel sheet from around 2 weeks post breast surgery, assuming the wound has healed normally. This should be continued up to 3 months post surgery. Scars take a long time to settle and can remain red for many weeks after surgery.

Best to follow your plastic surgeons recommendations for scar management.  I prefer to keep the incision covered with micropore  (paper) tape for the first 6 - 8  weeks and apply SkinMedica Scar Recovery gel on top of the tape twice a day.  Change the tape every 3 -4 days.  I find that most patients want to be proactive when it comes to scar management.  This starts with planning the incision, placement, how the incision, how the incision is repaired ( all out of the control of the patient) , the only thing the patient can do is either scar massage or better use the scar recovery gel as I described earlier.   

Listen to your doctor!!! She or he knows your surgical site better than any random stranger on the internet.  If you really feel you should do "something" then ask your surgeon when he or she thinks scar massage or a scar cream or tape would be appropriate. Personally, I find an incision in that area usually heals really well with minimal scar almost every single time.