Fraction questions can confuse a lot of students when phrased in this way, a good place to start therefore is to think of the number 9 as the fraction 9/1.
Now we know the rule that when you have a fraction you have to do the same to the numerator (the number on the top) as you do to the demoninator (bottom).
We know we want the denominator to be 3 as that is stated in the question, so to get from 1 to 3 we times by 3, so by doing the same to the top, we get (9 x 3)/3, which is 27/3, so there are 27 thirds in 9.