I am sorry to hear that you may have to undergo a third breast reduction.
Breast reduction surgery is safe and effective when performed properly. The goals of breast reduction surgery are not just to decrease the volume of breast tissue but also to improve the shape of the breast. Frequently, this involves lifting the position of the nipple back onto the chest if everything has "sagged" - a condition known as ptosis. Your plastic surgeon needs to move the nipple to its new position with its blood supply and nerve supply intact - otherwise you could end up with a numb nipple, or worse, no nipple (if the blood supply is completely interrupted by the repeat surgery).
It may be useful for your surgeon to get the operation reports from your previous surgery so the direction of blood/nerve supply to the nipple can be determined. A "last resort" scenario could be that your surgeon suggests a "free nipple graft" - which is where your nipples are removed at the beginning of the procedure and then replaced as a skin graft at the end of the procedure. If this were done, the sensation would never be the same, although the cosmetic result can be quite acceptable.
I hope this helps.