How do know if you can divide a number by 3. Is there a pattern?

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.

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