Star/Delta, but not as I know it.

[nike123]

For a constant supply line voltage, you can never alter the speed of a 3-phase motor unless we have a VFD control or dual voltage motor.

That is different than what you said here:

You can never change the speed of any 3-phase motors unless you vary the line voltage frequency or the no. of poles of the motor.

Any asynchronous motor have slip. That slip varies with motor power and required power for synchronous speed. If we aply power regulation to motor(by varying voltage or by pulse with modulation) we vary actual speed of rotation of fan no matter if it is 3 phase or single phase.