The co-op is offline only.
Anyways, from any area of the game, hit "Y" button to get to the menu.
Hit left on the joystick/D Pad to scroll to options.
Once at the options screen, hit "Right Bumper" to scroll right, on the options menu. On that screen (I can't think of the exact name), there is a controller selection area on that one screen. You can select each of the 3 players to be controlled by your selection of controllers.
I hope that explains it.
Back from the dead, I'm afraid.







