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The characteristics of traction network (TN) of the three-phase 3-kV traction power supply system (TPSS) are: three-phase conductors are flat layout (layout-1, layout-2); the material and cross-section of conductors are ferromagnetic and irregular, respectively. So the phase impedance matrix (PIM) is unbalanced and the closed-form formula for conductor impedance is hard to get. To address these issues, firstly, the PIM is derived, and the cause of unbalance is revealed. From the perspective of structure optimization, a new flat layout (layout-3) and an equilateral triangle layout (layout-4) is proposed. Then, to calculate the wideband frequency self-and mutual impedances of the special phase conductors, a frequency-and current-dependent closed-form formula based on 4-parameters equivalent tubular conductor (ETC) model is proposed. Furthermore, an equivalent TN circuit model at 50 Hz is established to calculate the receiving-end voltage and its negative sequence voltage unbalance factor (NSVUF). Finally, using these two indexes, the performances of the layout-1 to 4 are evaluated and compared. The results show that the conductor spacing and power supply frequency affect the two indexes much while the dimension of the power supply rail (PSR) not; layout-4 can completely balance the receiving-end voltage and improve its magnitude which can enhance the power supply capability.