This paper presents a multi-port isolated DC-DC converter for DC applications. A three-port structure is presented, characterized with full bidirectional power flow and simple control. Galvanic isolation is achieved by means of a multi-winding medium frequency transformer which is a part of a resonant LLC converter. To provide controllable power exchange with external DC ports, two out of three ports are equipped with additional bidirectional buck/boost stages. They serve to provide active power flow control, while the inner resonant stage provides galvanic isolation, operates in the open-loop and adapts its mode of operation based on the actual power flow. Both switched mode and rea-time hardware-in-the-loop simulations, with control algorithm deployed on the digital signal processor, are used to verify and demonstrate various operational modes.