how can AC be called single phase ,three phase..?

Answer 1

Single phase AC can be carried on one wire, with another wire to act as a ground. The hot wire carries a single sinusoidal voltage.

Three phase AC needs three hot wires and one ground. The three hot wires are just like the single-phase wire, except they are one-third of a cycle out of phase with each other (hence, three-phase). With a three-phase system it's possible to draw power evenly from the whole system without the zero-power interruptions of single-phase AC, and with a smoother overall power curve than two-phase AC (which is basically never used).

Answer 2

Single phase & three phase AC current is a lot like a motorcycle engine with one cylinder and one with 3 cylinders.
Each of the cylinders in a 3 cylinder engine pumps at a different time on each revolution. The 3 cylinder engine runs smoother. The 1 cylinder has a heavy "flywheel" to keep it rotating until the next push.
With alternating current the voltage (the push) goes from 0 to postive max, back down to 0, down to negative max. and finally up to 0 again.

Answer 3

AC is alternating current. Essentially single phase is generated in one set of coils or windings, three-phase is generated in 3 sets of coils, all wound on the same object.

When one output voltage is peaking, another is midway through the cycle and another is at it's lowest. When summed, the overall output is smoothed, instead of being a sinusoidal waveform.