My breast is opening up quite a bit 3 weeks post op? (Photo)

I'm sorry to hear about the complication you have experienced; Although understandably distressing to patients, these types of incision line healing problems tend to heal over the course of several weeks. Sometimes, the area may look worse before it looks better. Most often, even larger open wounds have healed by the time a patient reached the two months post op mark.
These wounds go on to heal through a process of contraction where the tissues heal from the sides towards the center of the wound; occasionally, removal of unhealthy tissue (debridement) and/or removal of exposed sutures, may expedite the healing process. Close followup with your plastic surgeon will be important; treatment regimens will vary from one practice to another.  Whether or not antibiotics are indicated will be a clinical judgment, made after careful physical examination.
 
Best wishes; despite the incision line healing problem it is most likely that you will be pleased with the longer-term outcome of the procedure.

Hello. Unfortunately sometimes after surgery a small wound can form along the incision. I recommend following all instructions your surgeon gave you regarding wound care, and to keep all follow up appointments as scheduled to ensure proper healing. This type of wound may take several weeks to heal, and sometimes requires re-excision of the wound at a later date. Also, foul smelling or purulent drainage may be a sign of infection. I recommend you follow up with your surgeon to ensure you do not require antibiotics or further intervention.