If you don't own Sonic 3 & Knuckles then you don't own a Sonic game.
Gameplay received a 10 because of how easy all 3 characters are controlled. All characters have their own "double jump ability" meaning if you hit the jump button twice Sonic will do the insta-slash, "Tails" will fly or swim, and Knuckles will Glide. Also Knuckles has the ability to climb on walls. The fact that all 3 characters can do the spin-dash is a great addiction.
Graphics received a 10 because at the time they were the best Genesis graphics around. Also in my opinion all 4 of the main character models are the best that have appeared in the series (including the 3D titles). The environments of each level also fit with the theme of the level.
Sound received a 10 because these are by far the Sonic music (besides Green Hill & Star Light) to ever appear. Alas like every other Sonic game for the Genesis the music does tend to get repetitive.
Overall Sonic 3 & Knuckles receives a 10 because its amazing replay value. Their are up to 14 levels to play through with Sonic having 14, "Tails" having 13, and Knuckles with 12. Also some players might have a fun time playing through again for all 14 emeralds. This is a must buy.