How to get Level Select on Sonic 3 Complete
In Angel Island Zone Act 1, hang on to the swinging things and then press the buttons in order. Left, Left, Left, Right, Right, Right, Up, Up, Up.
After you hear a ring sound, press START and then press A to go to the Level Select screen.
Sonic Origins Code: Up, Up, Down, Down, Up, Up, Up, Up (Title Screen)
To unlock Debug Mode, enter sound codes 01, 09, 09, 04, 01, 00, 01, 08 in order to unlock it.