Assuming you are a good candidate for breast surgery in general (from the overall health standpoint and the anatomical "limitation" standpoint), then additional revisionary breast surgery will likely be safe. Of course, patients should always understand that every additional operation does carry some risk. Weigh the pros/cons associated with additional surgery as you make your decisions. Best wishes.
By safe, I assume you mean both for you as a person and for your breasts. If your candidacy for your first two operations has not changed from a health point of view, then having another breast augmentation should not be any more risky than the first two. With larger and larger implants, there is more tension and stretch on the tissues making them less able to hold the implants up and allowing them to drift downward. At some point, there will not be another larger size to fill the space. At that time, you may need or desire to then downsize the implant and take out some of the extra stretched out skin with a mastopexy. Hope this helps.