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Your basic problem is that the motor needs some applied power to generate any EMF, but if you apply that power then its not an open circuit test.

For a true induction motor you're probably stuck.  You'll have to measure various parameters separately while the motor is being driven.  By measuring the voltage, current, and phase of each winding you can eventually deduce the same parameters that could have been measured more directly with an open circuit test.

For completeness, it is possible to sortof measure the open circuit voltage of a motor while being driven with three stator windings as you show, but that has permanent magnets on the rotor instead of relying on induction to create the rotor's magnetic field.  Use a drive scheme where each winding goes thru the sequence of positive, open, negative, open, etc.  There are 12 distinct drive phases in one magnetic rotation.  You can measure the generated EMF during the open phases.

One issue is that the open phase of each winding is during its EMF zero crossing.  Still, that doesn't mean zero the whole time.  Sensorless drive schemes rely on measuring the timing of the zero crossing.

Another means is to get the motor spinning fast, then watch all the open circuit voltages as it coasts to a stop.