what you are trying to do is to build a VFD (variable frequency drive). The way to do it is to rectify the incoming mains to give you ~700V, then use three H bridge output amplifiers to handle the PWM for each phase and use a low level PWM generator that has three outputs each with the required sinewave modulation and the required different phase relationship. Changing the modulation frequency will then control the motor speed.
I do not think that thyristors are the correct device to use, as once they are turned on they do not turn off until the incoming voltage goes to zero. i.e. they are incapable of "chopping" an incoming DC supply.
Frank