2 phase vs 3 phase power density (switched reluctance motor)
There is no such thing as 1/3 pole at time. All three phases a sine with a shift of 120 degrees. When you add all the currents you get a rotating stator current, which is DC. This rotating current generates a stator rotating magnetic field. 

The same principle applies to two phase motor. The phase currents are shifted by 90 degrees and this forms a rotating field.  

A two phase system requires 4 wires, two per phase, meanwhile a three phase system requires only 3 wires, since the sum of currents shifted by 120 degrees is zero.

The rotor of the machine won't notice the difference between two phase or three phase stator.