You can map it to the button bar or actually map to a button on a device. Actually the Shuttle Pro might be more useful for that.
One of the nifty things about FCP is that there are several was to do one task.
Click “empty space”
I find clicking on a empty area in the timeline to deselct the most awkward way. It can be havok if you’re zoomed out or “focused” at some other point in the time line where you have a small clip or transition selected elsewhere that you can’t see.
Making a button
If all you have is a one button mouse doing it on the button bar at lest gives you a place to click and a little visual feedback that you’ve completed the task.
Mapping to a key
Mapping to a single keys is easier than the coordination to hit 3 keys.
Mapping to a physical button on a device (mouse, shuttle pro, etc) is good too since finger and physical button are “right there.”