You could add together all the digits and then see if that is divisible by three. For example the number 345. 3 + 4 + 5 = 12. 12 is divisible by 3 therefore this number is too.
If your number is quite large, like 3 digit or higher than that, then calculate the sum of their digits, if they will be divisible by 3, then the number is divisible, otherwise not.
For example. - "11055" Here, sum of digits = 1 + 1 + 0 + 5 + 5 = 12 As 12/3 = 4 (Divisible)