How can RPM be measured in 3 phase induction motor?
An induction motor is asynchronous and it is therefore hard to measure the rotational speed based on flux. Because the magnetic flux on the stator does not match the speed or the rotor. It has slip.

Motor drivers are able to do it by pulsing short burst of DC on the stator windings whilst analyzing what comes back. This only works when it's not driving. This method is used for flying start. 

Perhaps you can measure it based on the harmonics measured by the flux sensor if you mount it on the rotor. But this is inconvenient since you'd need slip rings to get the signal out the electronics on the rotor.

The reliable industry method is to use a induction pickup on a trigger wheel.