Multi-port DC-DC converters are an attractive solution for highly flexible and efficient interface of different elements of DC grids (e.g., sources, loads, energy storage). This paper provides an analytic solution for the natural power sharing characteristics of a galvanically isolated three-port resonant DC-DC converter. A DC-transformer operation mode is considered, with two ports actively transferring power to a third port. The mathematical model reveals that the power sharing characteristics are tightly dominated by the resonant tank parameters, even though they are also dependent on the differential voltage at the input terminals and the system losses. The accuracy of the proposed model is verified by an experimental validation on a lab-scale prototype.