time to make a better post than powder puff katelyn
Level select: Quickly press Up(2), Down(2), Up(4), after the Sega logo fades and Sonic starts to appear, and before the screen flashes white. A chime will confirm correct code entry. Press Up and highlight the sound test option on the title screen and press Start. Pause game play and press A to restart, B for slow motion, and C for frame advance.
Debug mode: Enable the "Level Select" code. Hold A, then press Start when selecting the desired level. Press B to change into an object in the level, C to copy that object, and A to change the object. Information in this section was contributed by branksome3. --- "and that's why you can't spell rael without REAL, cause my man be keeping it!" óRoboplus
The Level Select code on Sonic 3 (w/o S&K) is probably the most annoying D-pad combination ever for a Sonic game cheat and you have to be so quick too! I tried it on Mega Collection Plus about a week and never got it. I managed to get the code to work quite often when I had the original cart for my MegaDrive though...