How is an AC signal offset?
There are 3 phase alternators that produce a 3 phase supply which is then transmitted over 3 or 4 wire systems.One first needs to understand how a single phase voltage is generated based on Faraday's laws of electromagnetic induction.The generated voltage is, among other things, also dependent on the (sine of the) angle between the magnetic field and the coil in which the voltage is induced.Now in a 3 phase generator, the the magnets and the coils are aligned (mechanically) in such a way that the angles are 120 degrees apart.Consequently, the instantaneous voltage of each phase offsets the other phases by 2π/3 radians.