Peering into the settings for the magic mouse i have found out what looks like support for 3 finger tap gestures for Dashboard and/or Expose control , namely :
“Dashboard” – three fingers tap
“Exposé All Windows” – three fingers tap to expose all window
“Exposé Application Windows” – three fingers tap to expose app window
“Exposé Desktop” – three fingers tap to expose desktop
These look like the options you would see in a dropdown list from which to select one action associated with a 3finger tap.
Currently you do not have a mouse system preferences option for 3finger , only “One Finger” and “Two Finger” sections are available for now.
Another thing to note is that a 1 finger swipe action is also referenced , and that “Mighty Mouse” still remains as as string reference , if you search for that in system prefs the mouse icon will be highlighted , this could show that apple was really in a hurry on this , sloppy or understaffed , either way here is the system preferences movie for the magic mouse:
Who knows if 3 finger tap functionality was planned but not finished or removed , but it was not entirely built and disabled either , because it can not be enabled as it is.
EDIT : I released a free app to enable adding middle click , expose , dashboard , desktop , spaces , both button holding , etc , the works in my app