Speed control of three phase induction motor by supply voltage control using AC voltage regulators?

No. Use standard utility AC, 3 PH voltage to feed a Variable Frequency Drive (VFD). The VFD is then wired to the motor. The output of the VFD varies average voltage and frequency in a semi-constant proportion to keep Volts/Hertz ratio the same to the motor. The operator can then adjust or select the desired speed from the VFD and the VFD will run the motor continuosly at that desired speed. If you were to control only voltage to the motor without adjusting frequency in proportion, the motor would burn up.