Sonic 3 & Knuckles is the combination of Sonic the Hedgehog 3 and Sonic & Knuckles by connecting both game cartridges with "lock-on" technology. It has all of the content from said games, as well as exclusive features. Originally, Sonic 3 and Sonic & Knuckles were intended to be released as a single game, but time constraints and hardware limitations resulted in this game being split into two separate titles.
Sonic 3 & Knuckles is the combined story of Sonic 3 and Sonic & Knuckles, first starting with the former's stages and continuing with the latter. It features Sonic, Tails and Knuckles as playable characters, and notably remains the only game to allow the player to upgrade the Chaos Emeralds into Super Emeralds that grant access to Hyper transformations. The game has rarely been ported in recent years, often speculated to be due to legal issues surrounding the music; a remake of Sonic 3 & Knuckles was featured in 2022's Sonic Origins with a slightly different soundtrack and improvements like widescreen support.