3 weeks post surgery, 300cc under muscle, incision under breast and scar healing indented. What can I do? (photos)

Having mild scar tethering a few weeks after surgery is common, and will most likely round out over time. Many surgeons intentionally tack the inframammary fold to stabilize the implants while the healing occurs, and this may be contributing to your appearance.  I wouldn't worry for now.  Wait to speak with your surgeon, but know that this is unlikely to be a permanent problem, and is generally a straightforward correction in the unlikely event that it does not improve with time and scar massage.  Good luck!

Many thanks for your question. The scar in the fold can appear indented following this area being secured to the chest wall. Be reassured that over time this settles and you should expect this to improve in appearance. Your final result following breast augmentation takes at least six months to become apparent. 
All the best,
Jonathan

Thank you for your question and photos.  It is still early following your procedure.  The indentation may be from trying to secure the inframammary fold.  The shape of breasts and scar will change further with time so be patient for now as it is still very early.