27016

If you always have one more than a multiple of three, for example 10, then when you add it three times, you will get three times the multiple of three, plus the extra 3. Therefore, it will always add up to a multiple of 3. You can do this with any numbers as long as you pick 3 of them, and they're all a multiple of 3, one more than 3, or 2 more than 3.