And this is the problem, Curtain Rod. Exactly correct.
Few argue that 3-man cannot be done, or even done safely.
If one is truly honest with oneself, I believe NONE of the operators (being the corporate knowledge and subject matter experts), at least after seeing how we roster, believes we will do it safely. Just like now crews will be cobbled together willy nilly without regard to rest, body clock, or anything you mentioned. Indeed what MIGHT have been a safe original crewing will have the standard and dramatic last minute changes with a smattering of whatever crew is available on whatever body clock.
Look at how we do reserve. Just like now, an individual on a 6 am to 12 pm (or later) reserve will be rung at 1030 am (with CC knowing the day prior of the roster change and new duty assignment) telling him he's on for a midnight duty ULR (or LRO). The 'balanced' body clock crewing the individual expected to see at showtime will wind up changing with him finding out at showtime that it's now 3-off clock with one on and he is now relief.
We've had a pretty good demonstration of how people are rostered on a whim over the last several months. The pilot remains the ONLY backstop in declaring unfit and preventing a fatigue related incident or accident. Moreover, it's my belief significant pressure exists not to refuse a duty (or particular type of duty) when one is not able to accomplish it safely and well rested in our present climate.