To illustrate the difference between single phase and three phase, imagine a lone paddler in a canoe. He can only move himself forward while his paddle moves through the water. When he lifts the paddle out of the water to prepare for the next stroke, the power supplied to the canoe is zero.
Now picture the same canoe with three paddlers. If their strokes are synchronized so each is separated by 1/3 of a stroke cycle, the canoe receives constant and consistent propulsion across the water. More power is supplied and the canoe moves across the water more smoothly and efficiently.