3 weeks post op tummy tuck, my incision reopened. Is this something to worry about or a sign of infection? (photo)

This type of wound should eventually feel well. I think it would be best if you check with your plastic surgeon to see how they prefer to handle this. Everything you are doing seems perfectly reasonable, but the wound may need to be cleaned up of some of the injured tissue and antibiotics may be helpful.

Hello! Thank you for your question and providing pictures, this problem is not uncommon, sometimes tension or reaction to the suture can cause this, be sure to keep the area clean and dry for a faster recovery, if its taking too long to close your surgeon may need to remove that tissue and close the wound again. Be sure to follow all your surgeons instructions and discuss treatment options.
I hope this helps, best regards