Controlling the speed of three phase ac motor is done by controlling the frequency of the power line supply,  since the motor is synchronized with the line frequency. Three phase ac motor speed controller is actually nothing more than three phase sine wave power inverter with variable frequency. The three phase power inverter is a complex circuit, but fortunately there is an integrated circuit chip for this purpose. Here is the schematic diagram of the controller circuit:

ac motor controller circuit schematic

For the final motor driver circuit, you can see the component values are shown only for OB2 channel, but the others are similar. This 3 phase ac motor controller use optical isolation, so you must be careful in designing the layout, to make a clear separation between low and high voltage network. This separation is useful for minimizing the risk of hazardouz electrical shock.