A number is divisible by 3 if the sum of all it's digits are divisible by 3.
Example :
Is 49 divisible by 3?
Let's see. 4 + 9 is 13, but it cannot be divided by 3. So
49 ≠ divisible by 3
Is 60 divisible by 3?
Let's see. 6 + 0 is 6, and it can be divided by 3. So
60 = divisible by 3
