My daughter is 15 years old and her best size is 40DDD. What treatment would you recommend?

Hello! Thank you for your question! I hope that these answers help in
making your decision. She suffers from juvenile hypertrophy of the
breasts, which is typically treated by breast reduction. It is not
uncommon to have the symptoms that you describe such as
neck/back/shoulder pain as well as infections/rashes and shoulder
grooving, especially in teenagers once her breasts begin to fully
develop. The decision of whether to proceed as well as the timing will
ultimately be yours.Once one begins to have the symptoms that
you state above, consideration for a surgical procedure to ameliorate
your symptoms, assist with self esteem, and allow you to get back to
physical activities in your youth should be done. Given your symptoms,
you would be an ideal candidate. You must first discuss with her and her pediatrician, and then consulting with a plastic surgeon for
evaluation and examination to assist you in deciding if this would be
the right thing for you. Your surgeon will also go over what to expect
as well as the risks and benefits of the procedure - it is a safe
procedure. Overall, her symptoms should be ameliorated almost
immediately and hopefully give you more self confidence and an increased
activity level. Your breasts may still continue to grow over the next
several years, but it is certainly reasonable to consider this at this
age with the issues that you are having. I hope that this helps! Good
luck with your decision!

I'm sorry to hear about the physical and (probable) psychosocial problems your daughter's disproportionately large breasts are causing her. 
It is probable that she is dealing with juvenile breast hypertrophy along with the physical and psychosocial consequences of this diagnosis. In other words, the breasts are too large for her frame causing  both physical and psychological distress.
In my practice, timing of breast reduction surgery is determined on a case-by-case basis. It is not too early to do your homework and learn as much as possible  about  breast reduction surgery and the potential risks and complications associated with  the procedure.   
As she learns about breast reduction surgery, make sure she reads about the potential risk/complications associated with the procedure. You may find the attached link helpful in this regard.  For example, unsatisfactory scarring is  one of the potential complications. Make sure she also understands that further surgery may be necessary in the future (for example if the breasts were to grow in size again).
On the other hand, breast reduction surgery is one of the most patient pleasing operations we perform and I think that for the right teenager (enough symptoms) it may be an excellent option (regardless of the age).

Sometimes breast  reduction surgery is covered through health insurance. The best way to obtain insurance coverage for breast reduction surgery involves some “hoops” to jump through. The more documentation you have (for example, from your primary care doctor, physical therapist, chiropractor etc.) the better when it comes to obtaining insurance “authorization” for the procedure.
This documentation and letter/pictures from your plastic surgeon will help you obtain authorization.
 When the time is right,  seek consultation with well-trained/experienced board-certified plastic surgeons.
I hope this, and the attached link, helps.

If your daughter's breasts are no longer growing, then perhaps she is a candidate for a breast reduction. If she wants them reduced of course she needs your permission to do so.