Welcome to the FX codes   Blue codes are old, they can be found everywhere or in a few places.   Red codes I've found in mags and sites but I can't get them to work. (they are probably fake)   Green codes- I don't have this game so I can't test it.   Orange codes are discovered by me, you might find them elsewhere but they were here first.
GENESIS
Sonic 3
Sound Test, Stage Select and Extra Options During Game - At Title press Up, Up, Down, Down, Up, Up, Up, Up(ring)go below Competition to find the Sound Test option, just select it to go to a Stage Select. When playing 2P in Stage Select, Press C to change the yellow numbers    0=Sonic    1=Tails    2=Knuckles Go to a normal game for the Extra Options When paused    A=Go to stage select    Hold B=Slo-Mo C=Frame Skip   Play at Night - In Stage Select, Hold C, Start   Put the sign through the ground - In Carnival Night Act 1 hold Down after beating the boss (this will not work with the Mega hit series)   Debug - In Stage Select, Hold A, Start   Tails in Debug & Super Tails - At Title enter Up, Up, Down, Down, Up, Up, Up, Up code, then start a normal game as Tails (you can use a cleared file) Pause and press A, now Hold A, Start in Stage Select, Tails is now in Debug, create and hit an item box for Super Tails   Animation Test - In Debug hold C, B   Weird Characters - After beating the final boss and the character jumps toward the screen enter and exit debug   White Fox Ending - Enter the Tails in Debug & Super Tails code and use Super Tails to beat the final boss