3 weeks post neck lift - Swelling progressively getting worse. Will my chin slim up again? What's wrong? What to do? (photo)

First, you should see your surgeon just to make sure there is not a hematoma  present. It is also advisable to wear a compression face wrap during the recovery phase. This may help move along the healing process.

You need to give it time. The surgical maneuvers were performed adequately as evidenced by your immediate post op photos. At this point, there’s no reason for the result to undo itself after 3 weeks. Apply compression wraps to reduce swelling and follow with your surgeon.

and though you may feel you've regressed, your immediate photos show what the anticipated outcome will be when all the healing and swelling is done.  You should certainly see your surgeon (not nurse) and see if there are any fluid collections that may benefit from aspiration.  Otherwise there is not much else to do except to be patient and heal.  Keep your surgeon informed of your concerns so you can get the needed reassurances you desire so you can focus on healing.