Sonic the Hedgehog 3 & Knuckles

Sonic the Hedgehog 3 & Knuckles (or simply Sonic 3 & Knuckles or Sonic 3 Complete Version ) is a platform video game which is the result of locking-on Sonic & Knuckles with Sonic the Hedgehog 3 using the former catridge's unique "lock-on" technology. It is essentially a large combined game of its two lock-on components which contains all Zones from the said games and also contains special features unique to this game. This would have been the original Sonic the Hedgehog 3 the developers intended but time constraints by Sega resulted in the game being split into two.

Gameplay
While the gameplay in Sonic 3 & Knuckles generally remains identical to its two components, there are new features exclusive in this game and there are a large number of gameplay changes present.

Zones
In Sonic 3 & Knuckles, the Zones of both Sonic the Hedgehog 3 and Sonic & Knuckles are placed together, featuring a total of 14 Zones.
 * 1) Angel Island Zone
 * 2) Hydrocity Zone
 * 3) Marble Garden Zone
 * 4) Carnival Night Zone
 * 5) IceCap Zone
 * 6) Launch Base Zone
 * 7) Mushroom Hill Zone
 * 8) Flying Battery Zone
 * 9) Sandopolis Zone
 * 10) Lava Reef Zone
 * 11) Hidden Palace Zone
 * 12) Sky Sanctuary Zone
 * 13) Death Egg Zone
 * 14) The Doomsday Zone

Super Emeralds
Super Emeralds are enhanced versions of the Chaos Emeralds. When the player enters their first Special Stage in Mushroom Hill Zone, they teleport to Hidden Palace Zone where the Chaos Emeralds are taken from them and transformed into gray (apparently depleted of power) Super Emeralds, thus beginning the Super Emerald quest. If the player does not have all of the Chaos Emeralds prior to reaching Mushroom Hill Zone, the acquired Chaos Emeralds will become Super Emeralds and the player is given the opportunity to acquire the first Super Emerald. However, the player will not be allowed to return to Hidden Palace Zone via Giant Rings until they collect the remaining Chaos Emeralds. If the player has not collected a single Chaos Emerald by Mushroom Hill Zone and goes to Hidden Palace Zone via the first Giant Ring, the player must exit the area by jumping on the teleporter in the bottom central part of the room.

Super/Hyper transformations
By collecting all seven Super Emeralds from the Special Stages, each of the three main playable characters (Sonic the Hedgehog, Miles "Tails" Prower and Knuckles the Echidna) will have access to more powerful forms. As with super transformations, the player must have collected fifty Rings and then press the jump button during a jump (as long as Sonic is not equipped with an elemental shield) to initiate the transformation. While under the transformation, one Ring is consumed per second and once the player's Rings deplete to zero (or once they complete an Act), the user will revert back to normal.

Changes

 * The player can play as Knuckles in the Sonic the Hedgehog 3 levels although Knuckles' opening cutscene from Sonic & Knuckles has been removed in favor for transiting from Launch Base to Mushroom Hill.
 * As such, Knuckles' levels are different from Sonic's and Tails' in most cases, with new areas and bosses that are generally more challenging than Sonic/Tails' bosses.
 * Knuckles does not face a boss at Carnival Night Zone Act 2 and simply has to jump on the Capsule switch to clear the Act.
 * It is still possible for Knuckles to fight the boss (piloted by EggRobo) that Sonic and/or Tails normally fight in their version. The transition from Carnival Night Zone to IceCap Zone with Knuckles still remains the same as with Sonic and/or Tails when he hops in the cannon, but when IceCap Zone starts, Knuckles will still be seen emerging from the teleporter instead.
 * The player can play as Tails in the Sonic & Knuckles levels (except for The Doomsday Zone) or have Tails follow Sonic throughout the entire game.
 * After the levels of Sonic the Hedgehog 3 are finished, the screen scrolls downward with the Death Egg falling rather than playing the normal ending animation and gameplay immediately proceeds to the levels of Sonic & Knuckles.
 * The Big Arm boss fight at Launch Base Zone Act 2 is omitted for Sonic and Tails.
 * Knuckles will still have to fight the Big Arm but to even things out, the Ball Shooter boss fight is omitted for him.
 * Game progress can be saved in all levels (including those from Sonic & Knuckles) using the save feature from Sonic the Hedgehog 3 (there are now eight rather than six save slots). In addition, any existing save files from Sonic the Hedgehog 3 will be copied to Sonic the Hedgehog 3 & Knuckles (a game completed in Sonic the Hedgehog 3 will become a Mushroom Hill Zone save file). This feature is missing in some later compilations.
 * Save slots now display information about the number of continues, lives and the Chaos Emeralds surround the player's character as the player collects them (the Chaos Emeralds are replaced with their correspondingly colored Super Emeralds when the player collects them). Save files imported from Sonic the Hedgehog 3 will retain the character, last zone and number of Emeralds, but will default to three lives and zero continues.
 * The game uses the sound bank of Sonic & Knuckles exclusively, which means that the BGM tracks that were different between Sonic the Hedgehog 3 and Sonic & Knuckles (which include the title screen, 1-Up, Invincibility, Act 1 boss, Knuckles' theme and end credits) use only the Sonic & Knuckles version.
 * All Act title cards now display the game title of Sonic 3 & Knuckles.
 * Passing a Star Post can now open up all three Bonus Stages, including the slot machine stage (20-34 rings required), the magnetic orbs stage (35–49 rings required) from Sonic & Knuckles as well as the gumball machine (50–65 rings required) from Sonic the Hedgehog 3.
 * There are slight differences in object and enemy placement in some of the Sonic 3 Zones as well (Launch Base Zone in particular).
 * The method of accessing Sound Test/Act Select and Debug Mode is different (see Cheats).
 * With Debug Mode enabled, the player can access any level with any character, even those that cannot be normally accessed (such as Sonic and/or Tails accessing Knuckles' Sky Sanctuary Zone and vice-versa or even Tails or Knuckles accessing The Doomsday Zone).
 * There are a total of nine different endings; each character (Sonic and Tails together have the same as Sonic alone.) has one with zero to six Chaos Emeralds, one with all seven Chaos Emeralds and zero to six Super Emeralds, and the third with all fourteen Chaos and Super Emeralds.

Cheats

 * Sound Test/Act Select: At Angel Island Zone, grab hold on a swinging rope and press the following in order: Left, Left, Left, Right, Right, Right, Up, Up, Up. (A Ring chime will sound if successful.) Then pause the game and press to return to the title screen. The Sound Test option should be available to select below "Competition".
 * Debug Mode: After accessing the Sound Test, play through either act of Mushroom Hill Zone and grab a hold on one of the manual elevators and press the following in order: Left, Left, Left, Right, Right, Right, Up, Up, Up. (A Ring chime will sound if successful.) Then pause the game and press to return to the title screen. Go to the Sound Test again then while highlighting any Act hold  before selecting the Act to start the Act with Debug Mode.

Re-releases

 * Sonic & Knuckles Collection (1997)
 * Sonic Jam (1997)
 * Sonic & Garfield Pack (1999)
 * Sonic Action Pack for PC (2000)
 * Sonic Action 4 Pack for PC (2001)
 * Twin Pack: Sonic CD and Sonic & Knuckles Collection (2002)
 * Sonic Mega Collection (2002)
 * Sega PC Mega Pack (2003)
 * Sonic Mega Collection Plus (2004)
 * Sonic Mega Collection Plus & Super Monkey Ball Deluxe (2005)
 * Xbox Live Arcade (2009)
 * Sega Fun Pack: Sonic Mega Collection Plus & Shadow the Hedgehog (2009)
 * Sonic PC Collection (2009)
 * Sonic Classic Collection (2010)
 * Virtual Console (2010)
 * Sega Mega Drive Classic Collection Volume 3 (2010)
 * Sega Mega Drive Classic Collection Gold Edition (2011)
 * Sonic 3 & Knuckles for Steam (2011)

Reception
Reception for Sonic the Hedgehog 3 & Knuckles was extremly positive, with many fans still regarding it as the best Sonic the Hedgehog game of the franchise.

Scrapped mobile re-release
On October 18, 2014 (the twentieth anniversary of the original release of Sonic & Knuckles and subsequently Sonic the Hedgehog 3 & Knuckles), Christian Whitehead and Simon Thornley (who developed the remastered versions of Sonic the Hedgehog CD, Sonic the Hedgehog and Sonic the Hedgehog 2) released a concept video showcasing a prototype of a remastered version of Sonic the Hedgehog 3 as a direct remake of Sonic the Hedgehog 3 & Knuckles for smartphone devices they have worked on for a while (see video below). However, circumstances prevented the development from proceeding any further.

As showcased in the video, the following aspects of the game were completed:
 * The title screen
 * Angel Island Zone with all enemies, gimmicks, bosses and intro/intermission sequences
 * All three Bonus Stages
 * Special Stages, including “Blue Sphere” functionality

Trivia

 * If using Debug Mode and a certain number of S Monitors are destroyed at once (most effectively with the Insta-Shield), then the controls of the character will be locked and by pressing Start the game will turn back into Sonic & Knuckles. This is the only way to access Debug Mode in Sonic & Knuckles.
 * Taking care to avoid as many Special Stages as possible, it is possible to finish the game with only the seven Chaos Emeralds, retaining their Super powers until the very end of the game. In this case the game endings will resemble the good endings from Sonic & Knuckles.
 * In the game's level select menu, there is a sound test menu. Interestingly, setting the sound test to 26 will cause the Sonic the Hedgehog 3 credits music to play, despite it not being used anywhere in Sonic 3 & Knuckles. Interestingly, the other tracks that differ between Sonic 3 and Sonic & Knuckles (1-up, invincibility, Knuckles theme, Act 1 boss, intro) use the Sonic & Knuckles version.
 * The "& Knuckles" part of the title spawned into a popular Internet meme in modern times.