3 years post-op - Neck dent and sagging (photo)

Thank you for posting your question here!My answer will be based on the limited information and pictures provided, for a detailed and bespoke plan please consult with your plastic surgeon.
From the attached images it appears that you have an element of cobra neck deformity , this happens when the platysma muscle is no longer connected in the midline ( either it was never brought together or the sutures broke or cut through the muscle) and when too much deep fat was taken out or lost. you also have some loose skin. I am sorry that you are having those issues , but rest assured that a revision surgery will likely correct the issues in question . I would encourage you to consult with your plastic surgeon in person to discuss your options. I hope this helps, best of luck!