100000 Points

Manage to beat a stage at with your time at exactly 9:59 and you'll net yourself an easy 100000 points.

Frame by frame

When playing a level after enabling the level select cheat, pause the game, then press the C button to advance the game by one frame.

Level Select

When the "SEGA" screen fades, quickly press Up-up-down-down-up-up-up-up. If you have done this right, you should be able to scroll down to "Sound Test" below "Competition". The ups and downs are very difficult in the short amount of time provided, but with practice, it is pretty easy.

All 7 Chaos Emeralds & Super Sonic

To get all 7 Chaos Emeralds without having to complete their Special Stages, first enter the level select and sound test code. Go to the Sound Test and play the following tunes in order: 02, 04, 05, 06. You will hear an emerald sound if the code is entered correctly. To get Super Sonic after entering the previous code, just select any level from the level select and start it. Once you acquire 50 Rings, do a double-jump to become Super Sonic.

Control Tails in a one-player game

Start a one-player game with Sonic and Tails as the characters. With Controller two, you can take control of Tails while also using Sonic.

Hidden Special Stage

On the level select menu, play sounds 01, 03, 05, 07. Highlight Special Stage 2 and press A + Start.

Infinite Lives

Get up to Launch Base Zone. Sound any of the alarms, so that the Kamikaze birds come after you. Charge up a Super Sonic Dash in between the alarm, but do not let go of the button. The birds will continually crash into you. After about 30 seconds, you will have gained enough points to get an extra life. Continue the process for as many lives as you want.

Level Select (When Locked-On to Sonic & Knuckles)

Start a game and go to Angel Island Zone Act 1. Go to one of the swings that you hang from and grab on. While Sonic is swinging, press Left, Left, Left, Right, Right, Right, Up, Up, Up. You will hear a ring if you entered the code correctly. Pause the game and press A to take you back to the title screen. Press Down twice to find the newly unlocked Sound Test menu. Enter it, where you can play all of the sounds/music in the game and warp to any level.

Access level select using Sonic 2

To do this, you must have a copy of Sonic the Hedgehog 2. activate sonic 2's level select (sound test: 19, 65, 9, 17)
Now, without turning the system off, remove the cartidge, and insert the Sonic 3 Cartridge. Reset the Genesis console, keeping the A button held down at all times. On the title screen, press down untill you see sound test.
Success! you now have an easy (but different) way to access the level select and sound test!

If you are afrade of this damaging your games, please, don't try it. I have never had any game be damaged by this trick.

BONUS: If you look at the level Icons, you will see the icons from sonic 2. If you look carefully, you will see the Icon for Hidden Palace Zone, a level that was "removed" from Sonic 2!

Sonic and Knuckles Mini-Boss Music

Make your way to the end of Act 1 of Hydrocity Zone (Zone 2). When facing the mini-boss, keep yourself underwater until the water warning music plays. Then jump out of the water.

The game should now be playing the mini-boss music from Sonic and Knuckles, which wasn't out at the time (the music was evidently included in Sonic 3 to make the backwards compatibility feature easier).