Locking this to Sonic The Hedgehog 3 increased the Zones to 14 instead of just 6, and made it one of the best Genesis games ever, and THE best Sonic game right above Sonic CD in my opinion. I remember Sega hyping this game for like months before it came out, and playing demo versions on kiosks, and on Sega Channel months before it was released... Sonic's popularity was probably in it's peak at the time of this games release.
The whole Sonic 3 + Sonic & Knuckles experience was the best!
It also took ages to get hyper Sonic and Knuckles, as well as Super Tails to get the true endings. If they had then mixed this with the time travelling, bonus stages, and music of Sonic CD with cartoon scenes I don't think I would of ever been able to tear away from the TV.
The popularity of Sonic & Knuckles when it hit was phenomenal! We had to travel a very very long way just to find a place we could hire it from since it was all hired out any where in my suburb, or the next one, or the next one, or the next one, etc.