In the present method for manufacturing a three-phase transformer, first three stacks of steel strips of individual lengths are formed for assembly of a magnetic core. In a stack, the length of each strip and the position of the strips in relation to one another in a longitudinal direction are selected such as to allow the magnetic core to bend tightly around windings mounted on the middle portions of the stacks, said middle portions constituting limbs and being characterized by a stepped cross-section which approximates to an elliptical or circular shape. Primary and secondary windings are mounted by means of the unsupported winding of a wire which is tensioned and exerts a clamping force on the magnetic core. Aluminium wire with ceramic insulation is used. The core is then assembled. First the ends of the strips of the middle stack are butt-joined to the ends of the corresponding nearest strips of the outer stacks to form regions of the magnetic core which act as yokes. Then the ends of the corresponding outermost strips of the outer stacks are likewise butt-joined to form regions of the magnetic core which act as yokes. The end joins on the yoke portions are situated in different places.