My boob gap is getting bigger. (Photo)

I understand your concern! I tell my patients who desire a closer cleavage to wear an underwire bra day and night as a splint (for months), so as they drop and settle with gravity, they stay close in the middle. This is also very important at night, as the breasts tend to fall out to the sides when you lie down. I advise a full coverage, non push-up, minimally padded underwire bra, similar to the ones at Victoria Secret or Soma. If a patient desires even more central cleavage, I have them use a racer back clip (you can get one on amazon) to be used with that particular bra. (it tends to work better than buying a racer back bra). Not all plastic surgeons agree with the use of an underwire bra for various reasons, so I would first check with your plastic surgeon to make sure he or she gives you the OK. 

Thank you for your question and photographs.  Proper surgical placement of breast implants is placement directly behind the nipple.  Your implants seem to be in good position.  The gap between your breasts is indicated largely by your anatomy.  My strong advice is to wait a full 4-6 months before considering the final result of your breast augmentation and discuss your result with your plastic surgeon.  Best of luck.

Everything appears to be healing appropriately. you need some advice on postop garments at this stage that can help to stabilise the implants. Things will change over the coming months and best to keep in touch with your PS during this time. 

You have quite big implants on a small frame, so they have to settle somewhere.  I would guess your implants are under your pectoralis muscle, and this muscle shouldn't be detached along the sternum as this would create visible rippling from the implant.  As your implants settle they will descend a little and fill out the side of your breasts.  I would bet that prior to your augmentation that you basically had skin on your sternum and that you are very thin.  Your breasts / implants will soften as they settle and you will be able to create nice cleavage in a bra.

Thank you for your question about your concerns with the "gap" between your breasts. You may notice changes as your recovery continues for the next several months. At 3 weeks post-op it is far too early to determine your final result. Based on your photo that you provided, you appear to have a very good result and your implants are in a good position. Any concerns that you have above the widening gap, I suggest that you discuss with your plastic surgeon for further guidance.