hi is there any simple analog circuit for generating pwm pulses for a three phase inverter

 

This is one of those things that is so much more easily, accurately, and inexpensively accomplished with a microcontroller that it just isn't worth trying to make one using analog components.

If you insist on using analog components, you could look up "ring oscillator", and build one for three phases, then use each phase to control three separate analog PWM circuits.