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...