What does this plot represent for 3 phase induction motor?
Mechanical losses are bearing friction and aerodynamic drag. Those losses are not influenced by voltage. Power lost to bearing friction is directly proportional to speed because the torque loss is not much influenced by speed, but power is torque multiplied by speed. Power lost to aerodynamic drag is proportional to the cube of speed because is is like the power to drive a fan. Some of the drag loss is literally due to driving an internal or external fan that cools the motor.
The fact that mechanical losses are constant tells you that the change in voltage as no effect on speed. Not much speed change would be expected if the voltage changes without changing the frequency. However changing the voltage does change the available torque capability. For that reason, it is likely that the plot represents on-load motor operation.
Iron loss in an induction motor is proportional to voltage. Increasing the voltage increases the motor's magnetizing current and the iron losses. That loss is represented in the equivalent circuit by a resistor that is nearly directly across the power line terminals. Only the small stator winding resistance and inductance is in series.