when sonic 3 released, it was unfinished, and it contained only half of what the devs wanted to put in the game. so, later, they released sonic & knuckles, a stand-alone game that contains the other half. but if you mount sonic 3 on s&k (the s&k cartridge had itself a cartridge slot), you get the full game that the devs wanted, sonic 3 & knuckles, which contains both halves.
when sonic 3 released, it was unfinished, and it contained only half of what the devs wanted to put in the game. so, later, they released sonic & knuckles, a stand-alone game that contains the other half. but if you mount sonic 3 on s&k (the s&k cartridge had itself a cartridge slot), you get the full game that the devs wanted, sonic 3 & knuckles, which contains both halves.
S3&K - the OG DLC expansion that actually delivered