In short, there should be always exist a certain angle between the stator coil field and the rotor permanent magnet so that the brushless DC motor can rotate. The process of the rotor rotation is the process of changing the direction of the rotor magnetic field, in order to make the angle between the two magnetic fields , when it reach a certain extent , the direction of the stator field coils must be changed. So how do know when the direction of the stator magnetic field should be changed? That would depend on the three Hall . You can think that three Hall shouldering a task to tell the controller when the direction of current should be changed.