LOL - if anyone had the answer to that one, they'd be making $ millions selling it. And many billions more applying the same cure to human greying
3 years is on the young end of the normal range for greying with boxers - but definitively still within normal range. Paying attention to diet certainly isn't going to hurt, and can have many wider benefits - but I don't think you're going to find a magic bullet in a fish oil (or any other) capsule. Greying seems to be primarily genetically determined.