When you add up the sum of the digits, it should be a multiple of three. 21 is 2 plus 1 equals 3, so since 3 is a multiple of three, 21 is divisible by 3
You can know if the number is divisible by 3 by adding the numbers up. If the sum is dividable by 3 then the original number can be as well. For example, the number 93. 9+3= 12 and 12 can be divided by 3. This means 93 is also divisble by three. If you do divide 93 the quotient is 31.
