I have 286CC implants but after three weeks, they look smaller than I wanted. What's the appropriate size for my anatomy?

Over the next 3 months your breast implant shape and size will change significantly.  The implants should drop beneath your nipple areola and look much better.Generally speaking 280 cc implants should increase your cup size by a factor of 2 in the bra you are wearing before surgery.  Of course you need to discuss this in detail with your plastic surgeon.

It is important that the plastic surgeon in the patient are in agreement upon the final breast size before undergoing the operation. There are sizing systems are very useful in achieving this agreement.

I'm sorry to hear about your concerns after breast  augmentation surgery.  Although,  online consultants will not be able to be of much specific help to you, it may be helpful for you to know that a significant percentage of patients ( early on after breast augmentation surgery) feel that they are “too small” or “ too big” only to go on to be pleased with the longer-term outcome of the procedure performed.   Therefore, best to evaluate the outcome of the procedure performed about 6 months down the line. Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation surgery ( regarding breast implant size/profile selection) is:1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.  
2.  Again, have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining which operation and/or breast implant size/type/profile will most likely help achieve your goals.
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or "C cup" etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.  The use of computer imaging may be very helpful during this communication phase.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery.  The use of temporary intraoperative sizers with the patient in the upright position are very helpful.  Best wishes; hopefully you will be pleased with the long-term outcome of the procedure; if not, it is likely that revision surgery will be helpful in achieving your goals.