Could blood donation after fat grafting cause my breasts to shrink/lose volume or blood supply?

Great question! At 3 months the fat that has survived isn’t going anywhere regardless of what you do. Donating blood would not affect the survival or blood flow to the grafted areas at any stage, and donation is safe at any time point.

Dear Graceful795662m
blood donation after surgery should have no effect on your fat grafting results. Fat viability depends a lot of surgeon technique. The fat has to be placed back into the buttocks as soon as possible so that it has a greater chance of survival. It has to be placed carefully so that not too much fat is placed in only one area.
Daniel Barrett, MDCertified, American Board of Plastic SurgeryMember, American Society of Plastic SurgeryMember, American Society of Aesthetic Plastic Surgery

No, donating blood 3 months after your surgery should have no effect on your fat transfers viability or size.

At three months, the outcome of your breast fat transfer is final, and the results are unlikely to change in any significant way at this point. Donating blood did not have an impact on the outcome of your procedure.
Best,
Mats Hagstrom, MD