Yup, you are right. That was kind of a trick question though, but yes, the answer is true.
If you just had sonic 3 alone, you never got to fight knuckles. BUT, if you inserted SOnic and Knuckles into sonic 3, then you unlock the remaining levels. In one of the unlcoked levels (also avalable in S&K alone as long as you are playing as sonic), you get to fight knuckles.
But even though S&K is a continuation of Sonic 3, I always considered them 2 seperate games
If you just had sonic 3 alone, you never got to fight knuckles. BUT, if you inserted SOnic and Knuckles into sonic 3, then you unlock the remaining levels. In one of the unlcoked levels (also avalable in S&K alone as long as you are playing as sonic), you get to fight knuckles.
But even though S&K is a continuation of Sonic 3, I always considered them 2 seperate games