I believe 3 finger gestures are used by VoiceOver so there's a possible conflict there, but VoiceOver is a system wide on-off setting so I don't know if that'd be a good enough reason not to use them as shortcuts. VoiceOver more or less redefines how you interact with apps anyway, so I'm not sure it'd bit a big problem, but Apple may want to avoid having too many different actions for different combinations of fingers.