I had neck & facelift 3 months ago. I already have sagging skin on the left side of my neck. What can I do? (Photo)

From what I can see from these photos, it appears too much fat was removed from your submental area and the anterior platysma muscle bands were not approximated. The only treatment (in my hands and with 34 years of experience) is to wait a full 6 months and perform a revision. This revision may possibly be accomplished through the submental incision but will be difficult to perform. It will require undermining and redraping of the skin, precise approximation of the edges of the muscle and even adding fat grafts. Stay away from any laser.

Thank you for your photo and question. You are still recovering from surgery and I would not recommend performing any surgery on the area again until you have fully healed (at least 6 months). However, once you are fully healed, you should consult with a board certified plastic surgeon that is very experienced with primary and revision facial surgery as they will be able to assess your situation better in person. I think that you would benefit from a surgical procedure called J-plasma (Renuvion) which is a way to tighten the skin from the inside by heating the skin until it shrinks. Again, talk to a surgeon who will talk to you about all your options. 

Unfortunately, it is difficult to make any type of assessment from the images you provided. It is possible you have the indentation under your chin because too much fat was removed in that area. Residual laxity in the neck could be attributed to different factors, such as not tightening the neck muscles sufficiently when the loose skin was excised. I encourage you to schedule a follow-up with your surgeon to see if additional treatment is required.Your surgeon may recommend you wait a bit longer to allow the area to heal completely before undergoing any more procedures. Make sure you are working with an experienced, board-certified plastic surgeon that has the qualifications necessary to provide you an optimal outcome. 

A combination approach would give the most improvement. Nonsurgical options such as Kybella, PDO NovaThreads, and Ultherapy do well in combination for skin tightening, but the results are mild compared to surgical options.
I would recommend a VASER Hi-Definition Liposuction with ThermiRF, Renuvion (J Plasma) and InMode BodyTite to contour and tighten the chin and neck. A neck lift can be performed at the same time, if you're a good candidate. I suggest seeing a board-certified cosmetic dermatologist to develop a personalized treatment plan that works for you. Best, Dr. Emer.

Though it is difficult to address the images that you have provided it does appear that two issues are taking place.  It appears that there wasan abundance of fat removed from the submental area and the platysma muscles are separated in the midline of your neck.  Your surgeon can perform a platysmaplasty that will tighten up the neck appropriately and alleviate that indentation underneath your chin.  

Thank you for sharing your question and photographs.  Although your pictures offer limited views it appears that you may have had too much fat removed from the undersurface of your chin, resulting in the indentation.  As for your skin laxity/excess this would require an in-person examination.  I would allow your results at least six months time to mature and soften and then revisit your surgeon to discuss your before and after images.  The decision on any additional work can then be determined.