Excel Investigation: Multiples of Three

Age 11 to 16 Challenge Level:
Download the Excel file.

Take a multiple of three, for example 741.

741 is 247 lots of three.

The digits in 741 add up to 12.

And 12 is a multiple of three.

Do all multiples of three have digit sums which are also multiples of three? If they do, why does that happen?