Risks of 3rd Breast Augmentation?

470 cc's is a pretty generous size implant.  I personally would not advise going any larger. When you have repeated operations and particularly when you keep getting bigger implants, the breasts tend to sag over time and the tissue becomes thin.  Go too large and  you may regret it.

The choice is based on your aesthetic demands. If you want to feel good in your clothes and be proud of your breasts, you may wish to undergo another surgery. Raffy Karamanoukian Los Angeles

Since your 1st augmentation did not work and the 2nd did not work then it is most likely that the 3rd augmentation will not work. Unfortunately, you have not provided any photographs. However, you may benefit from a technique called The Mini Ultimate Breast Lift. A circumareola incision is used to reshape your breast tissue creating upper pole fullness, elevate them higher on the chest wall and more medial to increase your cleavage. Aligning the areola, breast tissue and implant over the bony prominence of the chest wall maximizes anterior projection with the implant you already have. Remember the goal is to achieve beautiful breasts, not to put in progressively larger implants. Volume does not create beauty.

Best Wishes,

Gary Horndeski, M.D.

Size change is the number one reason for Reoperation after breast augmentation. Each time another surgery is done, more tissue is removed, there is exposure to infection risk, there is additional tissue trauma, and theoretical risk of capsular contracture. 

You have to weigh your discomfort over your new side versus the risks of Reoperation.