Before we start, controls for debug:

Xbox 360 Controller:
X: Reverse Gravity
A: Enter Debug Mode
C: Jump
While In Debug:
X: Change Item
A: Exit Debug Mode
C: Place Item
Keyboard:
Z: Reverse Gravity
X: Enter Debug Mode
C: Jump
While in Debug:
Z: Change Item
X: Enter Debug Mode
C: Jump

Okay, so you want to glitch the game to where you can play Sonic & Knuckles instead of Sonic 3 & Knuckles? Alright, I'll tell ya.

Step 1. Go to Angel Island Zone.

This is to enable Level Select, which is required to perform this glitch. You will need to do this regardless if you can access it now or not. If you know what to do, skip to step 2. then keep reading.
What you have to do is go through Angel Island Zone Act 1 and find one of the vines which you swing on. Once you do, perform the following code:
Left Left Left, Right Right Right, Up Up Up.

http://cloud-2.steampowered.com/ugc/597003505311199773/451E8629C06E97D70C736676C65DDD6514C1BB0B/256x0.resizedimage
The vines...if you didn't know what it was.


If your annoying partner tails' wasn't collecting a ring and you heard a ring chime, press start then press A (set to X by default on Xbox 360 controller, Set to Z by default on keyboard). Now you have level select enabled!

http://cloud-2.steampowered.com/ugc/597003505311247994/400AE19F2A1BCE8C534A00AC74F0DFFBA9BCBF78/256x0.resizedimage
Level Select

Step 2. Go to Mushroom Hill Zone.

Okay, now scroll down the menu to find Sound Test. This will bring you to a level select menu! Ah the memories...oh, anyways, now then go down to Mushroom Hill Zone (Act 1). This will let us go to the next step: Enabling Debug Mode.

Go find the first pulley that let's you go up by using Up and Down on the controller to go up. Then re-input the same code as before
Left Left Left, Right Right Right, Up Up Up.

http://cloud-2.steampowered.com/ugc/597003505311254477/E0F1B9C1423671F3E12DBE0C9A6D920E3C460CC1/256x0.resizedimage

I DIDN'T FORGET TO PULL THAT LEVER!

Now that our requirements are met, let's do Step 2! DOING THE GLITCH.

Step 1. Go to Angel Island/Mushroom Hill Zone

Either zone can be chosen to perform the glitch. The best choice is Mushroom Hill Zone, but I used Angel Island zone in the screenshots.

http://cloud.steampowered.com/ugc/597003505311350160/B508E1CB8CC507E25EDA07A2B73AA5A11EA66190/256x0.resizedimage
Debug mode once activated

Step 2. DOING. ZE GLITCH!

Okay, now then, once debug mode is activated, switch the item once. Depending where you are, you might see a corrupted sprite or a proper static monitor. Either way it's gonna work. Now then, place the item repeatedly until the timer says 1:80

http://cloud-2.steampowered.com/ugc/597003505311360278/36546804AEB83E2E7A6460EE651207D431D1EBB2/256x0.resizedimage

1:80

Now, exit the debug mode. Hit the monitors all at once (spin dash will work). WARNING: IF YOU HAVE HAD SEIZURES, LOOK AWAY. Now then, once the screen screws up, press start...and finally, S&K!

If you can't do it correctly or if you think I write walls of text instead of helpful info, have this video.