Sonic 3 & Knuckles is the game you get by combining S3 and S&K and can be seen as the true Sonic 3. Let me explain.
Sonic 3 was being made by Sonic Team. The game was too ambitious and it was decided to split it into two games, wanting to get the first part out by the Xmas time. Sonic The Hedgehog 3 was out in early 1994, missing Xmas anyway. Featured Sonic and Tails, had the first 6 zones with final zone made harder. Included competition mode, saving up to 6 slots. 16 Megabit.
And then they released Sonic & Knuckles by the end of 1994, 16 Megabit cart. Just Sonic and Knuckles in remaining 5 proper and 3 not-so zones.
But that's not all, as game had Lock-On Technology: you could insert another cart on top of it. Sonic 3 would give access to everything game had to offer, even upping save slots to 8. It did some changes to Sonic 3 levels, changed few music tracks, moved the disappointing S3 final boss to Knuckles only as regular boss.
You also could insert Sonic 2 and play as Knuckles in S2 levels. Sonic 1 gave access to procedurally generated Special Stages from S3. Putting other game carts worked same but you were given access to only one stage.
There was a native PC port but it replaced music with MIDIs. Yew!
The game that you have here is nothing more than emulation of Sega Genesis version of S3&K. While I am happy that people get to play Sonic 3 the way it was meant to be played, there should be ability to choose just S3, S&K or S2&K.
Now, technical part about Sega's emulation here:
The Sega Classic games that you purchase on Steam count as DLCs for "Sega Mega Drive & Genesis Classics" game that should appear in your library.
It has Bedroom HUB which is the one with many features yet lags for many and Simply Launcher which lacks Workshop and Online but at least it works just fine for everybody.
However, Simple Launcher has it's fair share of glitches as well. It can crash. And it does the second time you go to main menu, so always quit after saving there so it doesn't crash when you want to save next time!
Emulation itself, mostly sound, isn't that good but it does it's job. Also, yes, emulator supports quick saves.
As alternative, you can use external emulator to run games that you purchased. Sega kindly placed in all games that you purchased in "uncompressed ROMs" folder that program itself doesn't use, just change file extension to ".bin" or so. The file for this one being "Sonic_Knuckles_wSonic3.bin".
I also demand you to read digital manual of this game first. You can find it here on store page or go to "manuals" folder of game root and open "14 S3K_PC_MG_EFIGS_US_v6.pdf".
Has online (in Bedroom HUB only) and local multiplayer.
And yep, this game includes saving. In-game saving, not emulator one. Bedroom HUB works fine but Simple Launcher one is buggy:
You have to remember, you have to close emulator in proper way, otherwise it will not have them actually saved in files. That means that you have to avoid crashing it or closing in different way.
Even more, if you load emulator-side saves, the emulator will not see in-game saves and will not be able to do in-game saves either. Even if it makes look as if it does.
Death Egg from S2 crash landed onto the flying Angel Island, pushing it down to the sea. Knuckles, the guardian, has believed Robotnik that Sonic is coming to steal the emeralds. Time to finish Death Egg!
Fun fact: Knuckles is colored pink as NPC and red as playable char cus pallete limits.
It's one of the early games to tell a story without words, another great thing that S3&K has going. But at a cost of the wonderful abstract feel.
Core gameplay stays true to Sonic laws. Once again it's excellent platformer with one of the best physics with slopes. Press Down while running in order to roll as it will increase your speed while moving down the slopes and put you into attack mode too. Ring health system is still there, where you drop all rings on hit and have to recollect it, otherwise you will die on the next hit. There is Spin Dash: hold Down and press Jump once or more to charge up, shooting forward once you stop pressing Down.
This time each character has unique abilities when you press Jump in midair. Sonic can use shield active abilities. There are now three elemental shields with passive and active ability, each good in their own parts. Without shield Sonic does insta-shield that widens your attack radius and makes you invincible for a moment. Awesome pro move.
Tails learned to fly as part of gameplay. What a fox! And swim. Limited but fancy. Just like in S2 you can play a Sonic & Tails coop, where camera follows Sonic and he is the one that can die while Tails is a support, the second player being able to take control from AI anytime. Even better Sonic can now grab on flying Tails! Or Tails player can use this ability to try to ruin Sonic's day as anti-coop.
Knuckles not only can glide and climb the walls and has lower jump. He has Hard Mode campaign that takes place after Sonic and Tails. Same zones with occasional Knuckles route forced on you. Bosses are harder, some are different. Albeit his campaign is shorter.
Levels are much bigger as well. Programmers managed to squeeze in more out of console and each stage has new gimmicks. Two acts per zone and now there are minibosses! Sadly, I feel that each act lasts way too long. Some first time players may even hit 10 minute time limit. The game just doesn't have that arcade quality of beating it at evening that previous Sonic games had, thus I finally understand why some people prefer Sonic 2.
Protip: To pass the barrel in Carnival Night Act 2 jump on it and press Down and Up accordingly.
There are now simple bonus stages. Three of them. Just activate checkpoint with certain amount of rings and jump into stars above. Not a fan of them, pacebreaking, shields or rings are common anyway.
Protip: You can knock signpost around before it lands.
Once again there is the quest to get all Chaos Emeralds. Now you just have to find entrances that are hidden all over the levels. Special Stages are fun. You run around the looping grid without stop where you can turn by 90s degrees. Avoid red spheres and turn all blue spheres into red ones by passing over. What makes it fun is that if you turn borders of blue sphere rectangle into red ones then whole rectangle turns into rings!
Protip: If bounced to run backward just press forward to fix it.
Again, if you collect 7 chaos emeralds, you will unlock super forms unless Tails and thus getting better endings. You activate them by pressing Jump in air with 50 rings without shields. For the first time in franchise you also unlock extra zone if playing as Sonic (& Tails). And then you can collect 7 super emeralds to get even better forms, Tails included, and shinier ending.
It was Sonic Team's magnum opus on Mega Drive, with graphics being totally awesome. Especially new CGI-like Sonic sprites. A lot of use of parallax and special effects. Too bad Robotnik doesn't laugh anymore. Music is absolutely jamming too.
The game has versus multiplayer. This time you have to run through extremely short looping level 5 times, a race. Chars have different balance from main game. Sonic for max speed, Knuckles for traction and Tails for starting speed and can fly.
Sadly, not enough interaction between players, just single trap button in some levels. If you choose to have powerups then it will spawn 2 at the lap gates, immediately used. And objective is so one-dimensional: just get first. Not as good as S2 Versus. Weird physics too.
Match Race is just single level race. Time Attack is obvious. And Grand Prix where you play all 5 levels for the most wins.
Overall, one of the best.
