Okay...

In Maple Ontario Canada, I am the ultimate master of this game 9even though I probably suck compared to the real masters of the world), therefore:


Sonic three is the original game. you can be sonic or tails. In sonic and knuckles, you can be either sonic, or knuckles. Sonic 3 has a save game feature, sonic and knuckles does not. Sonic 3 has its own levels, sonic and knuckles has its own levels (same with special games and emerald rounds). When you combine them, you get Sonic three and knuckles which has:
All of the above packed into one, so it becomes a super game.



Why am i so good? I have just genesis, and that's all. no ps2, not even ps1 or 64. Just genesis. Yup. Hope i helped.

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I am 26 years old and I own practically every one of his games. Sonic 3 was actually the first Genesis game I ever got (I got a Nintendo in 1988, and didn't get a Genesis until 1993). To prove my Sonic "nerdyness" (Yes...and I'm sort of embarrassed of it) here is a list of Sonic games I own.


Genesis:
Sonic The Hedgehog
Sonic The Hedgehog 2
Sonic The Hedgehog 3
Sonic & Knuckles
Sonic 3D Blast


32X:
Knuckles Chaotix

Sega CD:
Sonic CD (still have the original box and manual)

PC:
Sonic CD

Saturn:
Sonic Jam
Sonic R
Sonic 3D Blast

Dreamcast:
Sonic Adventure
Sonic Adventure 2 w/ 10th Anniversary Golden Disc & Coin
Sonic Shuffle

Gameboy Advance:
Sonic Advance
Sonic Advance 2
Sonic Advance 3


Gamecube:
Sonic Heroes


Nintendo DS:
Sonic Rush
Sonic Rush Adventure


Wii:
Sonic Unleashed



XBOX360:
Sonic Unleashed
Sonic the Hedgehog 06 (Just to let you know, I didn't pay for this gawd awful game. But it's a Sonic game none the less. So I had to get it)


XBOX:
Shadow the Hedgehog (Again, a game I did NOT pay for. But it's a Sonic game, and blah blah blah...)

PSP:
Sonic Rivals
Sonic Rivals 2



Yup. Now about the story behind Sonic & Knuckles, it's basically episode two of Sonic 3 (hence why the upcoming DLC Sonic 4 isn't called Sonic 5). When STI and Sonic Team (the half that didn't work on Sonic CD)were developing Sonic 3, they ran into a few snags. For one they were rushed, and half of the game was done (which is why in level select for Sonic 3, you see the names of levels from Sonic & Knuckles). That and 34 megabit cartridge (cartridges with a lot of space what were crazy expensive at the time) with NVRAM (the abiltiy to save) was too costly. So Yuji Naka decided it was best to split the game in two.

The decision was taken to split the game in half, giving Yuji Naka and the other developers more time to finish the second part--and splitting the high cost between two cartridges. When people locked on Sonic 3 and Sonic & Knuckles, the two roms join one another and you get Sonic 3 & Knuckles.
This game is basically the biggest 2-D Sonic game of all time (unless Sonic 4 is bigger).

Sonic's story takes place after Sonic 2. Knuckles story picks u after you beat the game with Super Sonic (which is why Knuckles fights the Egg-Robo that came out of the rubble of Death Egg when you beat the game with Sonic. Eggman isn't in Knuckles' story. And I guess Tails story is just an extra thrown in).