The output of 3 phase machine is always greater than single phase machine of same size. The output will be approximately 1.5 times than single phase machine.
For transmission of electrical power three phase supply requires less copper or less conducting material than that of single phase system for given volt-amperes and voltage ratings. Hence 3 phase system is more economical compared to single phase system
Single phase machines are not self-starting machines. On the other hand three phase machines are self-starting due to rotating magnetic field.
Three phase system gives steady output and also ensures that a single-phase can be derived from it itself.