Conventional full-bridge and half-bridge LLC resonant converters are no longer able to meet the growing power rating requirements and bi-directional operation needs of today’s on-board chargers. By placing symmetrical resonant tanks on the secondary side of the transformer, the CLLLC resonant converter enables bi-directional power flow while achieving a wide voltage gain range. In addition, the three-phase CLLLC structure has the advantage of increasing the output power and decreasing current ripple. However, magnetic integration techniques must be employed to mitigate the increase in size and cost associated with additional magnetic components. In this paper, a symmetrical three-phase CLLLC DC/DC resonant converter using an integrated transformer is presented and the gain characteristics of the converter are evaluated and analyzed. Finally, a 20 kW DC/DC converter prototype using SiC MOSFETs is built and the feasibility of the theoretical analysis is verified.