Multi-port converters have attracted special interest in applications such as self sustaining residential homes and buildings with multiple energy sources and energy storage. In this paper a constant switching frequency series resonant three-port bi-directional dc-dc converter is proposed. The converter uses a three-winding transformer with two of its windings connected to H-bridges through series resonant circuits. To control the output voltage and to share the power between the two sources, two control variables are proposed. One of them is the phase shift between the fundamental of the outputs of the H-bridges and the other between two legs in one of the bridges. Sinusoidal analysis is presented and equations for output voltage and input port power are derived as a function of the phase shifts. It is observed from the analysis that the power can be made bi-directional in either of the source ports by varying the phase shifts. Switching frequency is chosen such that the converter operates above resonant frequency and hence aid in Zero Voltage Switching (ZVS). A design procedure to select converter parameters is explained. Simulation and experimental results of a prototype converter are presented.