A three-phase layered power flow solution has been here developed, which can handle distribution transformers with various common winding connections in unbalanced system of multi-voltage levels. In the proposed solution, a sensitivity constant incident matrix between node voltages and injection currents was derived based on path matrix, then a simple direct algorithm in impedance form was utilized to resolve the three-phase unbalanced power flow. An efficient layered algorithm was obtained by dividing a large and complicated distribution network into several small and simple networks (called layers) to overcome the large size problem of incident matrix for a complicated distribution network. Then the size of incident matrix has been reduced and the space requirements have been greatly decreased for the entire system. The exact three-phase transformer models are critical for distribution system analysis. The models for several kinds of main connections have been developed in this paper. The transformer can be considered as one special layer during solving the power flow. The validity and effectiveness of the proposed method and the transformer models have been testified by the IEEE 4 and IEEE 34 test feeder.