According to the official story, Sonic 3 was meant to include the "& Knuckles" content from the beginning, but it was cut due to time constraints. Sonic 3 was pushed out as the "vanilla" package, with the "& Knuckles" expansion pack (lock-on cartridge) coming out eight months later. I personally think the "& Knuckles" content ends up detracting from the core Sonic 3 experience, and would have been fine if it never saw the light of day. "& Knuckles" added the Mushroom Hill zones onwards, this is where the game starts to drag the most and see quality loss in level design in my opinion. If they really needed to have the "& Knuckles" content added to complete the package, it would have been a better experience start-to-finish if they had removed the Sandolopis zone, which is the lowest quality zone of the "& Knuckles" campaign.
I'm not a fan of most of the minigames present here, which include a slot machine minigame, some weird orb minigame I always died within seconds of starting, and a bubblegum machine minigame that gives you powerups and oneups. The bubblegum minigame is the best of these non-Chaos Emerald minigames, and it's also the only non-Chaos Emerald minigame that was available in the original Sonic 3 experience. "& Knuckles" brought the other two - that quality reduction I was talking about... these three minigames are accessed by having certain ring requirements when hitting a checkpoint, if you have enough rings it puts a warp above that checkpoint to use. There's also a blue-sphere minigame here for Chaos Emeralds, which you access by finding Giant Rings hidden through the game. This is a great part of the game, one of my favorite things of the Sonic franchise, especially the music. It's pretty simple, you're on a 3D plane and you have to collect all the blue spheres, there are pinball spheres that knock you back, there are also red spheres. When you pick up a blue sphere, it turns into a red sphere. If you touch a red sphere, you fail the minigame. If you pick up all the outside spheres of a blue sphere batch (ie. the outside 8 in a batch of 9), all the spheres in that batch turn into rings, and you get credit for the blue spheres that were turned into rings. This makes it easier to navigate the level as there are fewer red spheres to accidentally run into when navigating the 3D plane, which gets more difficult the longer you hunt for all the blue spheres as your character continuously increases in speed.
Getting all the Chaos Emeralds gives you access to Super Sonic, which you are then able to activate on the first act following when you acquired Super Sonic mode. Get 50 rings, then press the jump button while in the air (it won't work if you have any shield powerups). One ring is consumed every second. When you get to the "& Knuckles" content (starting with Mushroom Hill), you can enter an Emerald room very early on that takes all your Sonic 3 Chaos Emeralds. Now you need to hunt for Giant Rings all over again, but now you're getting Super Emeralds for succeeding at these minigames. These add a new sphere, an orange one, that acts as a jumppad. I found all of these easier than the original Sonic 3 Blue Sphere minigames, but it's nice having more of them since it's a great minigame. When you get all the Super Emeralds, you get access to the best ending in the game, as well as the ability to go Hyper Sonic. This is the same as Super Sonic, except he can double jump, which also triggers a screen-wide instakill attack on non-boss enemies. Tails and Knuckles also have Super and/or Hyper modes in this version.
You can play as Knuckles and have different routes exclusive to him through the same zones, since he can glide and climb walls. I didn't bother completing this because I didn't enjoy the "& Knuckles" content of the game very much at all, so didn't care enough to replay all of it as him just to see how much has changed. Just did enough to see if it was different, and it is, though from what I saw, a decent chunk of the acts are still the same. In the future I'll be sticking to the base version of Sonic 3.
There's a mod to let you play the original Sonic 3 since it wasn't included officially in this Steam release. This used different music for some acts, and had slightly different enemy and powerup placement in some areas, though it isn't a matter of one game being harder or the other being easier. The changes made to the "& Knuckles" version of Sonic 3 makes some of those areas easier and some of them harder, while the original version has those same areas being harder in those zones and easier in the other.
Just like Sonic 2, there is a coop (Sonic and Tails) and competitive race mode.
Michael Jackson supposedly wrote some of the music for this game and then wasn't credited for reasons we'll never know for sure (my money's on Sega wanting to distance themselves from him after certain things about him came to light). The OST here is very high quality, certainly better than Sonic 2 and better than 1 in a lot of ways, they're good jams, but I think Sonic 1 had more memorable music. The melodies simply stood out more.
http://steamcommunity.com/sharedfiles/filedetails/?id=1350592768
