The size of a three phase motor is smaller than that of a single phase motor of the same rating. Three phase motors are self starting as they can produce a rotating magnetic field. The single phase motor requires a special starting winding as it produces only a pulsating magnetic field. In single phase motors, the power transferred in motors is a function of the instantaneous current which is constantly varying.
Hence, single phase motors are more prone to vibrations. In three phase motors, however, the power transferred is uniform through out the cycle and hence vibrations are greatly reduced. The ripple factor of rectified DC produced from three phase power is less than the DC produced from single phase supply.
Three phase motors have better power factor regulation. Motors above 10HP are usually three phase. Three phase generators are smaller in size than single phase generators as winding phase can be more efficiently used.