This hint links to a third party driver for the Apple Magic Mouse. It's still in beta but works great. It works for all operating systems from 10.4 on. Their Preference pane/driver offers greater functionality for the Magic Mouse than the native Apple driver.
With this driver your Magic Mouse has four buttons instead of two, each button can be assigned a different task for single finger clicks and for double finger clicks. There is a three finger click option but it is restricted to only the front of the mouse or the back as three fingers are too wide to fit in only one button space. Each of these click combos can also be assigned a separate function for soft taps instead of clicks. You can even remap the virtual edges of the buttons if, for example, you need a wider space to click because your right finger tends to wander and could use the additional room.
[crarko adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100805135831788