What numbers can be divided by 3?

The divisibility rule for 3 is as follows:

If the sum of the digits of a number is divisible by 3 then the number is divisible by 3.

For example, the number 21 is divisible by 3 since 2+1=3, and 3 is divisible by 3. The number 49 is not divisible by 3 since 4+9=13, and 13 is not divisible by 3.