If all the digits in a number add up to a number divisible by three, the number itself is divisible by three.
For example the number 711: 7 + 1 + 1 = 9. Nine is divisible by three, therefore 711 is divisible by three.
yeah 3 9 15 21 27 and so on (every other multiple of three) But if you have a large number, like 755253, and don't have a calculator handy, then use sum of digits to determine if divisible by 3. Then...