The number of multiples of 3 from 3 to 999 should, in principle, be 999/3 where we count the 999 and the 3 (note that division is inclusive, while subtraction is exclusive).

That would make 999/3 = 333, but we are told not to count the 999 or the 3, so that makes 333 - 2 = 331 multiples. That gives us A.

Check this with smaller numbers to simplify the process: if we count multiples of 3 from 3 to 12, that's 3, 6, 9, 12, or four multiples. Same as 12/3 = 4 analogous to above.

Then if we don't count 3 or 12, that's 4 multiples - 2 = 2.