First perform the Sound Test code. Now, at the sound test menu, press Start to go to the character selection screen. Choose your character, and before the fight starts, hold Select. If done right, you'll go into the options screen where you'll be able to select a secret 3rd Speed option.
Easy Story Mode
First enter the Vs Mode and select the character you would like to play as in the Story Mode. Then select the second player (it can be anyone). When you are asked to select the handicap, put it all the way to the right, so the 1st player has all the advantage. Now, win the fight. Once it's over, select "Exit" to go back to the main menu. Now select Story mode and pick the same character you used in Vs Mode. If done right, you'll character will retain the huge handicap and the enemies will be much easier.
Player Modifier
At the Vs Mode character select screen, press the X button 1 time to change the 2nd player into a CPU controlled character. Press X a second time to change both players into CPU controlled characters. Press X a third time to return everything to normal.
Sound Test and Sprite Viewer
At the title screen press Up, Up, Down, A, A, A and Start. You'll now be at the main menu. Perform the following actions to access the secret modes.
Effect
Code
Sound Test
Highlight Story mode, hold Select and press Start.