3 phase VFD/rheostat for motor
The ONLY is going to be use a VFD.  To vary the speed of a three phase motor, you need to change both the frequency and voltage.  That is exactly what a variable frequency drive does.  Using a rheostat to vary the voltage will cause the torque to drop off, but won't change the speed until except that it will drop when loaded due to reduced torque.  It will also heat up more than it should.