RyanTScott :
OK cool. What I'm most concerned about is the MBs ability to consistently run at 100% load for extended periods of time. Does that change your answer?
No. Any of those three 300 dollar plus mobo's better be able to run a cpu at 100% load for extended periods of time. Just watch all the mobo's temp sensors and the cpu temp sensors at first to make sure everything is okay. You are an early adopter for X399 and threadripper so keep an eye out for news of common problems that may crop up.
The VRMs seem to all be pretty similar at a glance, so I see nothing obvious that would make any of those three mobo's better than one another when under a heavy prolonged load.