Transformers with three or more windings are called multiple winding transformers, and are used to interconnect three or more circuits with different voltages.
A multi-winding transformer costs less and is more effective than an equivalent number of two-winding transformers and in addition to power transformers, this configuration also exists in small transformers used in electronics.
In rural distribution transformers, there are two 127 V windings connected in series. Low-power circuits can be powered at 127 V, while motors can be powered at 254 V.
Single-phase systems, especially ground return monofiles, are built at lower costs because they have simplified construction and lower maintenance and operating costs.
Charges in rural areas are generally so small that it is seldom necessary for three drivers to accommodate their carrying capacity. Simplification of construction allows lower costs.
In transformer systems, transformers usually have three windings, as for example in a substation that reduces the voltage from 230 kV to 138 kV, there will be a third winding in 13.8 kV connected in delta.