Sonic 3 & Knuckles: Master Edition is a ROM hack of the original Sega Genesis game. 'Master Edition' implies a compilation of enhancements and improvements, potentially incorporating elements or fixes from other ROM hacks or fan projects. Created independently by fans, it's not an official Sega product.  It will likely improve the overall experience while preserving the core gameplay.

Sonic 3 & Knuckles: Master Edition

Sonic 3 & Knuckles Rerouted (v2)