Sonic Classic Collection Cheats

In the first zone (Mushroom hill) Jump on one of those lifty things (The one that you press down on) and press left, left, left, right, right, right, up, up, up. You will hear a chime. Now, go get a game over. The game will restart. at the title screen, hold Y and press start. Now, you will be able to choose any level, including sonic 3 ones. Have fun!

At the title screen, press A twice, then Up, Down, Left and Right. You should hear a ring chime. Hold down Y and press Start. Debug Mode is now active, as indicated by the score tally being made of letters and numbers. Y & C are still jump while B changes Sonic into an object which you can spawn by pressing the C button. Press Y to cycle through them and B to change back to normal. Be warned that over-using this will cause temporary slow-down and could cause problems.

I don't know if you also have to have the level select code on, but I've done this a few times.