We currently have two fractions: y / 1 and (y - 3) / 3.
In both cases, we are only concerned with the denominator. Yes, the numerator will change when we change the denominator, but that will not affect the answer to this question.
As such, we need to find the least common multiple of 1 and 3. That is 3. Therefore, the common denominator is 3.
Hope this helps!