To access both a level select and the game's Debug Mode, press Up, Up, Down, Down, Up, Up, Up, Up on the screen between the SEGA logo and the Title Screen (where it turns white and Sonic spins toward the screen). You'll hear a ring collection sound if you did it right.
To find the level select on the title screen, simply press "Up" to reveal it after you do the debug mode code. To access debug mode, Hold "A" before going into any file/level, then press "Start" and hold until play begins.
While in debug mode, you have access to the following:
- Infinite Time
- Debug Coordinates (In place of score)
- Sprite Counter (In place of time)
- Placement Mode (See below for details)
- Pause Options
- Press "A" to go back to the title screen.
- Press "B" to enable slow motion.
- Press "C" to enable frame advance.
Press "B" to enter placement mode. From there...
- Press "A" to move forward one object.
- Hold "A" and press "C" to move backward one object.
- Press "C" to place the object.
TH In addition, if you die by running into an enemy or falling into a pit, press "B" to enter placement mode, and then press "B" at a safe spot to revive yourself. However, trying to place an object in this death state will freeze the game.
Additionally, do not enter debug mode at any point where Sonic goes into "rotation" mode (Such as while standing on the red and white cylinders in Carnival Night Zone) or the game will crash. 
Sonic and Knuckles Zones Edit
During Development of Sonic the Hedgehog 3, it was originally going to be one big game, but due to time constrants, the game had to be split into 2. The level slots for the Sonic and Knuckles are still in the game. On the level select, you can see 3 unused zones in the game, Flying Battery (which was originally going to be in between Carnival Night and IceCap), Mushroom Valley (early name for Mushroom Hill) and Sandplois. The other Sonic and Knuckles Zones are still in the coding, and can be accessed via, game genie/PAR.
Flying Battery Zone Edit
The Zone is still playable. It can be accessed by using game genie for either Act 1 or Act 2. There's a lot of stuff in the game for Flying Battery, such as a save icon for it, the tracks for Act 1 and 2, Title cards for both levels, the bosses for both acts, and the manual mentions enemies from both acts (Blaster and Technosqueek). While both levels are still playable, it uses messed up IceCap tiles, and there doesn't seem to be much of a floor.
Mushroom Valley Edit
The zone itself still exists in the coding, however, unlike Flying Battery, the level isn't playable (if accessed, it'll crash the game on the title card, with the music playing). There is no save icon, and the title card glitches once it comes on the screen.