When Sonic 3 and Sonic & Knuckles are locked together, they produce one of the greatest Sonic the Hedgehog games ever made. This page largely focuses on the changes between playing the games together compared to playing them individually.
Our extensive guide starts off with when Sonic and Tails defeat Eggman at Launch Base. You don't fight Big Arms from Sonic 3 in this extended version, which is nice. You are then moved on to S&K's Mushroom Hill Zone, but SEGA have added an extra scene to the beginning of it. It features Knuckles being very sneaky and hiding something.
Investigate when Knuckles leaves, and you realise its a giant ring. If you've already got all of the Chaos Emeralds from Sonic 3, you'd be wondering why the heck this would be here. The ring is actually multicoloured and when you jump into it, it takes you to the Hidden Palace Zone! Why? Well, if you've got all of the Chaos Emeralds, you can start to collect the SUPER Emeralds, which go alongside the Master Emerald.
You might think that this is totally pointless, but get them all and you can turn into... HYPER SONIC, HYPER KNUCKLES AAAAAANNNND...... SUPER TAILS! Yes that's right, all you Tails fans can now rejoice that you can now control him in Super form.
For Knuckles fans, the big draw for this locked-on experience is that you now can play the Sonic 3 stages as the gullible red echidna. Unlike Sonic & Tails' story, you fight BOTH bosses on Launch Base, but hey, Knuckles's story is shorter, so there :D. Anyway, after Launch Base, a robotic 'Eggman' drops a bomb on the Launch Base, and Knuckles get caught in the explosion throwing him into the Mushroom Hill Zone. Again, you get a special scene here if you are playing Sonic 3&K, with a giant ring. When Knuckles enters the ring he is taken to the Hidden Palace, just like Sonic and Tails.
Seeings as Tails does not get a part in Sonic & Knuckles, when you do get to play him in Sonic 3&K, there's certain areas that are inaccessible to Sonic and even Knuckles. This is because of his flying ability. Knuckles can glide and climb, but he can't hover in mid air and not lose altitude, only Tails can do this. You can use this to your advantage in Hidden Palace Zone, where you can fly high into the sky and find a platform full of these goodies, with two more ring monitors and a 1-Up nearby that.
All this hype about special hidden areas, you'd thought we'd forgotten about Sonic, eh? Well, think again, because Sonic is the star of the show, and the reason that The Sonic Stadium is here. In fact there WAS a level I erm, 'forgot' to mention in S&K. That level was the VERY last one, and that was The Doomsday Zone. After you defeat Eggman on the Death Egg, if you have all of the Chaos Emeralds and Super Emeralds, you can enter this final battle between HYPER SONIC and Eggman as he tries to get away with the Master Emerald into Space.
Thankfully, air isn't a problem for Super, or even Hyper Sonic. This level has Sonic transform into either Super Sonic or Hyper Sonic and using your provisional 50 rings, you must try to stay as your alter-ego for as long as possible until you complete the level, by collecting rings. Run out of rings, and you transform back and fall to your death. Defeat Eggman for the last time and peace will be finally restored!
Now, this is what it's all about - this was the main intention of Sonic 3, however the remaining part of the story had to be carried over to Sonic & Knuckles, but boy was it worth it! All the goodness of Sonic 3, playing as Knuckles to open previously inaccessible pathways, playing Sonic & Knuckles levels in a two player co-operative mode, with the addition of Tails, and the final apocalyptic ending with Hyper Sonic! This... is a legend, not a game. Believe it. Oh, and buy it.