Three phase power generation
Short: 

3 phases give you reasonably constant energy delivery across one cycle, not too too much implementation infrastructure compared with the two wires of DC or 1 phase AC and much less than 5 phase. $ and practicality lead and the industry follows.



Longer:

I don't know the formal answer (although I'm sure that Garglabet does) BUT it was probably not "some point in history (and if it was it would be easily enough identified)), but more likely to be "market forces building an "edifice" that was not liable to be overturned. 

DC and 1 phase AC require two conductors.
For a reasonably balanced feed, 3 phase requires 3 conductors for transmission over distance. Neutral is something you can derive remotely as required.  3 phase gives you a true rotating field for rotary machines - induction motors and others) unlike 1 phase.
Assuming odd number of phases are liable to be selected for a range of reasons, 5 phase requires 5 wires. And 5 sets of insulators per location. And extra hassles with transformer design. For minimal gains.
Throw in $ and practicality and  3 phase looks likely to be the winner that it proved to be.