Assuming you use a 3-phase direct on-line contactor ....
Out of interest, why would you want to vary the voltage on a 3-phase motor
I suggest you consider using ready-made SSRs rather than roll your own with opto triacs and power triacs.
PS - also see the bit in the paper I linked to which explains why you should NOT use zero-crossing SSRs with 3-phase induction motors.
The resistor needs to be rated at 415VAC
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