When you plug Sonic 3 into Sonic & Knuckles, that’s what Sonic 3 was intended to be. But they couldn’t finish it in time, so they split them into separate games. That’s why when you play Sonic 3 & Knuckles, you don’t just play Sonic 3 as Knuckles, you play through both Sonic 3 and Sonic & Knuckles (as opposed to Sonic 2 where you only play through Sonic 2 levels as Knuckles).
When you plug Sonic 3 into Sonic & Knuckles, that’s what Sonic 3 was intended to be. But they couldn’t finish it in time, so they split them into separate games. That’s why when you play Sonic 3 & Knuckles, you don’t just play Sonic 3 as Knuckles, you play through both Sonic 3 and Sonic & Knuckles (as opposed to Sonic 2 where you only play through Sonic 2 levels as Knuckles).