3 weeks post op breast reduction, my wound has opened. Do I need stitches? (Photo)

I would not recommend further surgery at this time unless there is an underlying reason why the wounds have come apart. This wound separation at the T junction can occur and it sounds like you have been given appropriate advice. It is best to try and heal this conservatively. Often this does heal and the scars settle without the need for any scar revision.

Wound dehiscence is not uncommon after a large breast reduction. The majority of wound issues occurs exactly where yours are. The area commonly refered to by plastic surgeons as the "inverted T", is an area of tension that is prone to wound separation. 

Your wounds should be kept clean   Usually a regimen of local wound care will be recommended by your surgeon that should allow the wounds to heal by themselves over time. Suturing of these open wounds is very rarely used to close them. Inflammation and swelling in the area create unfavorable conditions rarely assisting in wound closure. It is almost like trying to stitch through wet toilet paper. Antibiotics are rarely indicated with clean open wounds. Debridement, or gentle surgical cleansing of the wound may be performed to prevent or deter colonization of the wound with bacteria. 

Dilgent wound care , close follow up with your surgeon, and patience will be keys to your recovery. Should you require any future scar revisions, this will probably be addressed by your surgeon after 6months. 

Try to be positive. In time you should have a very nice result.