Compare tap changing transformer and three winding transformer?

Tap changing transformer (Two winding transformer) is used for transformation of One voltage level into another.

Three Windings are required Where you require three levels of voltage.These windings are 1.HV (High Voltage) Winding 2.IV (Intermediate voltage) Winding 3.LV (Low Voltage) Winding The main task of voltage transformation is performed by HV winding & IV winding, while as LV winding is used for minor purposes as auxilliary winding.