While there wasn't an official prototype for Sonic the Hedgehog 3 released until November 2019, here's some theories about the game earlier in development.
Some things of Sonic 3's Prototypes:
- The game would have two plots branching off each other (much like the final), With the first plot being much like that of the final, the main difference being that Robotnik already has the all the Super Emeralds. And after you beat him in Launch Base Act 2, the Super Emeralds get scattered. But while Sonic was going through zones, Robotnik got the master Emerald. So then Sonic has to get the Master Emerald back from Robotnik.
Some versions of prototypes of Sonic 3
- Early prototype, uses Sonic 2's resources, has early soundtrack.
- Early Prototype, uses an early version of Sonic 3's engine, has early soundtrack
- later prototype, has all 17 zones, has final's soundtrack
- later prototype, Launch Base is the last zone, Flying Battery is still present.
- later prototype, level designs have been revamped, Flying Battery and Azure Lake has been removed.
This version of Sonic the Hedgehog 3 is an early version. It uses the final's music (although some tracks are in an early state).
- Sonic's graphics look different, they look like his Sonic the Hedgehog 2 counterpart, but his colour is the shade of blue used on Sonic in the final, and some of his animations are similar to Sonic 1 and 2 on the Game Gear/Master System.
- The unused sleeping animation and whistling animations are used.
- The more earlier prototypes had a different save system than later versions, unlike the later versions, the more earlier prototypes had it where, you can select "New Game" (which starts a new game), and if you leave the game, your progress is saved, and you can continue the specific zone and act via "Continue" option on the title screen. (however, anytime you pick "new game", it'll erase the data that was saved under "continue".)
- In addition, the title screen also lacked the Competition mode from the final, but also has an options menu similar to Sonic 2's, where you can pick to play as either Sonic, Tails, Sonic/Tails, and Knuckles. Along with a sound test, which includes the Sonic 2 method of accessing the level select.
- Later versions changed the SRAM system to the Save system of the final.
- Competition Mode was called "2 Player" earlier in development.
- If the Sonic and Knuckles Cart is placed underneath it, the game will read it as "Sonic 2", due to having the same serial code as Sonic 2.
- Knuckles is playable without lock-on, although there are some differences with Knuckles:
- The game doesn't end at Sky Sanctuary, instead, Knuckles goes through the game up to the final boss.
- Eggrobo isn't fought in Knuckles story, unlike the final.
Angel Island Zone
- It's possible to drown in Act 1 after beating the boss during the results part where it counts the time and your score.
- Originally, this zone was after Mushroom Valley, but before Sandopolis.
Marble Garden Zone
- This Zone features elements not featured in the final, such as switches that if stepped on, opens a pathway by crumbing a certain area.
- There's an oversight in Act 2's boss, where Tails can get tired in the air during the battle, so then he'll fly downward (like he does in the other levels), however, the bottom of the screen isn't a pit, which then, Tails will fly back up with his tired animation, and then drops sonic as soon as the animation for picking Sonic is done. After which, the animation will go lower and lower the bottom of the screen, to where Sonic will fall out of the top of the screen and fall to the bottom, and tails will be missing.
Carnival Night Zone
Flying Battery Zone
- Act 1 originally had 2 parts, midway through Act 1, Knuckles would arrive and press a switch, which would cause the player to fall into the second part of the level, however, due to a bug in some versions, it instead takes the player to Flying Battery Act 2.
Azure Lake Zone
- Act 2's music would later be used for Carnival Night Zone Act 2's music in the PC version of Sonic 3 and Knuckles.
Launch Base Zone
- The level layout and designs for some objects are completely different.
- Launch Base Act 2 is completely different from the final, in the prototype, after you beat the Boss of Act 1, then the Death Egg's Rockets start up and start launching up. Sonic grabs onto the rockets when it launches up, and the level takes place on the Death Egg.
- In much later versions, much like the final version, this zone becomes the final zone of the game.
Mushroom Valley Zone
- Act 2's boss is different, being much more based off the Palmtree Panic Boss from Sonic CD, but with big Mushrooms replacing the bumpers that were on Sonic CD's version. Along with this boss having a move where it can fly up into the air, and a aiming riticle will appear near sonic and follow him, after it's on Sonic, the boss will slam into the ground. Much like Sonic 2's final boss.
Endless Mine Zone
- Level design based off of Mystic Cave Zone from Sonic the Hedgehog 2.
- Originally after Angel Island Zone, but before Marble Garden. later versions swapped this zone's placement with Hydrocity's.
- Has 3 acts instead of 2.
Balloon Park Zone
- Act 2's music was later used for an unused song in The PC version of Sonic 3.
Chrome Gadget Zone
- Act 2's Music is the same as Azure Lake Act 2's music.
Lava Reef Zone
- Has 3 acts instead of 2.
- Hidden Palace is Present, however, if accessed from the level select, it'll call itself "Lava Reef Act 4".
Sky Sanctuary Zone
- The Knuckles version of Sky Sanctuary is absent.
Death Egg Zone
- Has 3 acts instead of 2.
- Music is completely different, the track it uses for Death Egg Act 3 would later be used for Launch Base Act 1 in the PC version of Sonic and Knuckles.
- The Final Boss is completely different.
- Act 2 features many maze-like elements not used in the final, and even includes the infamous spike trap from Mystic Cave Zone from Sonic 2.
- The spike traps are littered throughout some areas of the level, upside down and rightside up.
- Act 3 has a "revisiting levels" gimmick, where if a player goes through a certain transporter of the area, it'll take the player to that zone.
- In the area of Act 3 with the re-visiting levels (where Sonic/tails/Knuckles can pick a level to go to.), it uses the music used on the continue screen, Competition mode results screen and the Blue Sphere "no way" screen from the final.
- Doomsday is completely different.
- Background is different.
- The Gumball Machine, Slot Machine and Pinball bonus stages are present in the game.
- The Gumball Machine is mostly complete, although it is very buggy in many ways.
- The Slot Machine, is half-complete, the purpose of the bonus stage is very different, and the bonus stage is also very glitchy at times, due to design.