Three-wire Versus Four-wire
Three-phase circuits occur in two varieties: three-wire and four-wire. Both types have three energized ("hot" or "live") wires, but the 4-wire circuit also has a neutral wire. The three-wire system is used when the loads on the 3 live wires will be balanced, for example in motors or heating elements with 3 identical coils.
The neutral wire is essential when there is a chance that the loads are not balanced. A common example is seen in local distribution in Europe, where each house is connected to just one of the live wires, but each house's neutral wire is connected to one common neutral. When neighbouring houses draw unequal powers, the common neutral wire carries a current as a result of the imbalance. Hence electrical engineers work to make sure that the power is divided equally, so the neutral wire carries as little current as possible and therefore wastes little power. Obviously it is statistically easier to produce a good balance when supplying power to a large number of houses, so any large imbalances tend to be confined to small localities around a few houses.
The '3-wire' and '4-wire' designations do not count the ground wire used on many transmission lines which is solely for fault and lightning protection and does not serve to deliver power.
Read more about this topic: Three-phase Electric Power