It is a desired solution to integrate photovoltaics (PV) and battery into the high-voltage dc bus using high-gain three-port converters in renewable energy systems. Aiming at the limitations of conventional three-port converters—including restricted voltage gain, difficulty in achieving soft switching, and high voltage stress on semiconductor devices—this paper proposes a novel ultra-high voltage gain three-port converter topology based on a three-winding coupled inductor. Only one magnetic core is used so that the power density of the converter is effectively improved. The lower voltage stress and soft switching performance of semiconductor devices enable specifications with lower conduction losses to be selected, which can reduce system losses and improve efficiency. Based on the port power relationship, the proposed converter can achieve smooth switching between different operating modes. The topology and operating principles of the converter are analyzed in detail, and then the port voltage relationship, voltage/current stress, and control methods are analyzed in detail to guide parameter design. Finally, both simulation models and experimental prototypes were developed with PV input voltages ranging from 20 V to 40 V, a battery voltage of 48 V, an output voltage of 400 V, and a rated power of 400 W, validating the effectiveness of the proposed high-gain three-port converter and its control strategy.