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.
While an engineer at Xerox, my father modified an electromagnetic calculator to turn it into a form of stored program computer (similar in form and function to the Olivetti Programma 100). While experimenting with the calculator he found a way to crash it: divide 10 by 3.
He got a patent for his electromechanical computer invention, for which Xerox paid the standard nominal fee of $1, but it went nowhere.