Sonic 3 has three different final bosses for Sonic, which depends on how you play the game. If you play the game when it's locked on to Sonic & Knuckles (or if you play Sonic & Knuckles on its own), then you'll battle against Eggman's mech in the Doomsday Zone (or Great Eggman Robo in the Death Egg Zone if you didn't get all Chaos Emeralds). But if you play Sonic 3 on its own, you will instead fight Big Arm. This boss is still present in Sonic 3 & Knuckles, but only during Knuckles's playthrough for some reason. While I think Great Eggman Robo is a pretty good boss, I really don't like the Doomsday Zone. 50% of the battle is just collecting rings and the fact that you're invincible during all of it doesn't help much either. As a result, I usually prefer playing Sonic 3 on its own (when I want to play the game as Sonic) so I can fight Big Arm, who I think is the most enjoyable final boss when playing as Sonic.
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