Divisiblity Rule for 3

The rule for 3 is you add all the digits in the number. If it is 3 or a multiple of 3 then it is divisible by 3.
For Example:

Number 3456

3+4+5+6= 18

18 is divisible by 3 because it is a multiple of three. 3, 6 ,9 ,12 , 15 ,18