Are all large motors three-phase

Yes, most large motors are three phase. Usually single phase motors only go up to 10 HP. Larger than this and the wire size and contactors used to control the motors becomes extreme. For example a single phase 10 HP motor on 115 volts draws 100 amps. The same single phase motor on 230 volts draws 50 amps.

The same 10 HP motor on three phase system draws the following amperage; 230 volts is a 28 amp draw, 460 volts is a 14 amp draw and 575 volts is a 11 amp draw.

As you can see the higher the voltage becomes, the smaller the wire feeder size is needed and much smaller size contactor can be used to control the motor.