The big advantage to 3 phase is that motors can be smaller for the same horse power. Just as a single phase AC motor is smaller than a similar DC motor, so a 3 phase motor is smaller than a single phase. Airplanes use electric motors with even more phases where weight is even more important then size. In factories, it is common to have many high loads, and most of them use 3 phase motors and 240 or 480 volts. I once managed a small factory where almost everything was 480 3 phase or pneumatic. We had 3 phase motors ranging from 1/4 horse to 600 horsepower. I wouldn't even want to think about a 120 volt single phase 600 horse motor, or the conductors it would take to supply it. Most of the lighting was 277 volts pulling power from one phase of the 3 phase. The phases were 480 apart, but only 277 above ground. It is also easy to reverse a 3 phase motor, just switch any 2 leads. I even selected a vacuum cleaner that ran on compressed air because we weren't wired up to supply enough single phase 120 volts to run a large shop vac.
