Question 1: If I enable the midi clock out from the H9 Core, how do I set it up for the other 2 channels to recieve the midi clock?

MIDIclock is not channel dependent, so you should not have to do anything.

Question 2: How do I set it up to be able to update the synched tempo (across all 3 devices) with new tap tempo set by the H9 Core? 

Sorry – don't undestand the question.

Question 3: Is the H9's tap tempo footswitch robust enough to handle frequent taps?

Anything will wear out in time, or break if abused, but the switch is fairly rugged and can easily be replaced.