The magnetic circuit characteristics and operating equations of transformers with three different core structures: three-phase banked, five-limb, and three-limb are utilized. A numerical method is proposed to fit the circulating current in the delta windings using the zero-sequence current and voltage on the wye side. Through this approach, a deeper understanding of the characteristics of current in the delta windings and magnetizing inrush current in transformers with different core structures is gained. To validate the effectiveness of the proposed numerical method, software simulations and field waveform recordings are conducted. The results show that for three-phase banked and five-limb transformers, the differential current in each phase winding corresponds directly to the magnetizing inrush current of that phase; however, for three-limb transformers, the differential current in a phase winding is not directly correspond to its excitation current. Further theoretical analysis and simulation verification indicate that during no-load closing from the wye side of transformers with these three types of core structures, the difference between the line currents of any two phases on the wye side can be regarded as the difference in magnetizing inrush current between the two equivalent magnetic circuits. This finding provides a new basis for identifying the magnetizing inrush current of three-phase transformers with different core structures.