By plugging Sonic 3 into the top of Sonic & Knuckles using the latters unique "Lock-On" technology you can replay the game as Knuckles. Some of the levels contain ares that are only accessible using Knuckles leading to speculation that this game was designed with "Lock-On" technology in mind, or (More likely) that Knuckles should have been present from the start, only to be dropped later in development.