How Long Do Thigh Lift Outer Dissolvable Stitches Take to Dissolve and What is the Best Product for Cleaning Them?

I am uncertain exactly what suture your surgeon use and why they would have to stay in for three weeks.  Typically I do not use external sutures but when I do, I would remove them after approximately two weeks.  If your surgeon has left in outer sutures, I am sure they have a good reason. 
I suggest using hydrogen peroxide, which is typically used to dissolve any blood around the suture.  I believe the presence of blood stimulates bacteria and causes secondary infection. Hydrogen peroxide would be a very good product to keep the suture and suture line clean.  It may weaken the suture mildly but not significantly to break or remove the suture. This should not affect the quality of your thigh lift result whatsoever.
Congratulations on your weight loss surgery.  It is very important for you to continue compression for at least 3 to 4 months after your surgery as well as to apply silicone gel and silicone strips to the scar to allow for a flat scar.

Thank you,
J. Timothy Katzen, MD, FACS

It's difficult to know exactly what type of dissolvable suture was used during your thigh lift closure. In most cases, these wounds are closed with absorbable sutures that are buried beneath the surface of the wound. These sutures typically last for four to six months and are used to minimize tension on the closure. This, hopefully, decreases the potential for spread of the scar.Occasionally, these sutures work their way through the skin before they can resorb. It's important that these sutures be removed to avoid the development of infection around the stitch. With appropriate care, this should represent a minor problem with no long term consequences.It's not unusual for patients to develop crusting and scabbing along these wounds. When this happens, cleansing with soap and water is usually adequate treatment providing there are no signs of infection.If you're concerned about post-operative wound care, consultation with your plastic surgeon is appropriate. Your surgeon should be able to address your concerns and alleviate your anxiety.