What are the disadvantages of single phase electricity?

Single-phase power transmission requires a live and a return wire, so power is lost due to current flowing through two wires for each (there is one) driving EMF.

If you add two more at 120 and 240 degrees (one and two thirds of a period), then you get three-phase power. Now, the sum of all currents at either end is zero. When one phase's voltage is high, the resulting current travels down that conductor and returns via a combination of the other two; but these other two currently have a voltage causing that exact same current (split in varying proportions between them) to flow, meaning this current can be considered not to be a "return" of the first phase. You could consider it getting a return conductor "for free".