Reach level 3-1 in two player co-op mode. You can now enter a three player co-op story at the same point from the menu. Use 3 on the keyboard and do not use your game controller if one is active. Then only use two of the Toms to finish puzzle 3-1, one on each side of the barrier. Once you have completed this, keep pressing [Enter] until the level selection screen for level 3 appears again. You can then exit the three player keyboard and resume the co-op story.