By your use of the word macro I assume that you are using VBA within one of the office products. It may help to know which one you are using.
As for the question you could use MID$() to pull each digit from the number in question
You can then add the numbers together
You can use the MOD operator to see if the sum is evenly divisible by a given number.
For any number (expressed in base 10), if the number is divisible by 3 then so is the sum of the digits - and vice versa. So you could simply determine whether the number itself is divisible by 3.
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