Reduction genioplasty and sagging?

Considering the 3mm setback you are looking for, it is unlikely to do that. A word of caution though, if your intention of the surgery is to set the chin back only 3mm to release tightness of the submental area , I would be very careful. Give your surgery time and the tissue will soften up. In my opinion 3 mm is unlikely to have any impact on soft tissue tightness.

It will probably not cause an actual chin pad sag (as that remains attached to the bone) but may cause increased submental fullness as that is where the soft tissue redundancy could occur. But for a 3mm change the likelihood of that occurring is less than if it was totally reversed. But can always be address concurrently with the bony setback by a small submental tuck.