As with the first two parts, the work is dedicated to the development of the theory of transformers and identifying its new features using the concept of magnetic vector potential. It is proved for the idealized shell type transformer that a characteristic surface exists inside the primary winding in which the vector potential does not depend on the load. In opposition to the official theory, it turned out that not the «overall flow of the magnetization in the magnetic», but the magnetic flux covered by the characteristic surface of primary winding is supported constant in the transformer. The condition of active power transmission is the presence of a phase difference between the fluxes covered by the characteristic surfaces of the primary and secondary windings. A new type of the 4T-shaped equivalent circuit is obtained, with magnetic fluxes and transformer topology displayed on it. In case of short-circuit, the flux separation line in windows passes only within shorted winding. At the same time the individual parts of the magnetic circuit are overexcited in relation to their state when idling. These results are not compatible with the dogmas of the official theory and raise the issue of creating a new transformer theory with the development of more grounded standards on their durability to short-circuit needed to improve the construction of transformers at the design stage and, ultimately, to stop the observed increase in their failures. A physical justification of the necessity to increase the normalized impact short-circuit current ratio from 1.8 to 1.9 has been presented.