Im sure this is about some higher end math than I dont understand, but in day-to-day mental math just

1. multiply by 3

2. divide by ten

3. If more accuracy is needed, add 10% to the answer.

Of course, you can always add up all the digits and see if it's divisible by 3. If not, add/subtract to make it divisible by three, divide it and then correct for the borrowing.