The paper considers three-winding transformers with two parallel-connected sections of the primary winding and one secondary winding. This design is used in flash-butt welding transformers. The article provides a method for calculating currents, active power and losses in the windings, based on the equivalent circuit of a multi-winding transformer. The equivalent circuit of a three-winding transformer is based on its replacement with two two-winding transformers. All parameters of the equivalent circuit can be measured. The relationship between active power and winding losses has been established, which underlies the indirect method of measuring winding losses. The analysis of the accuracy of the technique was carried out on a model FEM transformer. To clarify the technique, vector diagrams have been constructed, showing, in particular, that the active power in one of the primary windings during a short circuit can be negative (the angle between the current and voltage vectors in the winding exceeds 90°). Relationships between the parameters of two-winding transformers in this case are obtained. The results of calculating the currents and active powers in the three-winding transformer of the KSS-04 welding complex for pipe welding are in good agreement with the measurement results.