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)
In short, This number would be divisible.
Hope this helps!