Can I override the action MacOS 10.8 performs for the triple-finger-tap gesture? MacOS looks-up the word under the cursor in Dictionary.app when you triple-finger-tap. Instead I would like to open a new tab in Chrome and Google that keyword. Is this possible?
Mike is right, Better Touch Tool can do the exact same thing you want. I was really lost when I upgraded to 10.7 and all its new gestures : BTT allowed me to restore my old 10.6 gestures.
Download Better Touch Tool for free here.

Then open the Better Touch Tool preferences. 


Select Gestures
Then be sure to select Global in the side bar. That way the new gesture will work in every app
List item
Click on Add New Gesture
Select the gesture to override : it should be Three finger tap or Three finger click
Then select the appropriate action : there is a predefined Search selected text with Google
Done ! You may have to disable the three finger gesture in System preferences.