The first time once you get Gohan back on Earth. Beat Buu as Super Saiyan Gohan. When it's over he will be a Super Saiyan 2. In order to unlock Super Saiyan 3 Goku, just unlock every character and their form (What ever you hear you DO NOT need Evil King Piccolo to complete this) and Goku's Super Saiyan 3 form should be available on the last fight.