Can I override the action MacOS 10.8 performs for the triple-finger-tap gesture?

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.

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

BetterTouchTool is an application that lets you assign configure gestures and trigger actions from your Trackpad. I'm not sure if it will allow this specific case but you should definitely be able to get something close.