Sonic 3 opens all its secrets with a single code, but it goes in at an awkward moment: on the SEGA logo screen, after the logo fades and Sonic starts moving, but before the title screen appears. The window is about a second wide. When it lands, a ring-collect sound plays — then press Up on the title screen, SOUND TEST appears, and through it you reach the level select.
CodeEffect
Up, Up, Down, Down, Up, Up, Up, UpOn the SEGA logo, unlocks level select and sound test
On level select: choose a level, hold A and press StartDebug mode — object placement, infinite time, coordinate readout. Keep A held until the level loads
On level select: hold C and press StartHighlights high-priority background tiles
Sound test: play 02, 04, 05, 06All Chaos Emeralds; an emerald chime confirms
Sound test set to 07, then hold A and press Start on “Special Stage 2”The hidden eighth Special Stage with the grey emerald
C on the level select screenCycles the Competition characters (00 Sonic, 01 Tails, 02 Knuckles); controller 1 sets the left one, controller 2 the right
On real hardware the code often fails — the SEGA logo screen carries a lot of lag frames, so it usually takes a few attempts. And mind two things. First, the popular “Left, Left, Left, Right, Right, Right, Up, Up, Up” rope-swing code on Angel Island belongs to Sonic 3 & Knuckles locked together, not to Sonic 3 on its own. Second, in debug mode do not enter places where Sonic’s sprite rotates (the red-and-white barrels in Carnival Night, for instance) — the game crashes.