The switching speed of next-generation power devices based on wide-bandgap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), are more than ten times those of conventional silicon insulated-gate bipolar transistors (Si-IGBT). This may increase the frequency range of electromagnetic noise produced by pulse-width-modulated converters. Further, the operating frequency ranges of noise filters are limited by the parasitic components of passive elements, so it is difficult to realize a high-frequency (HF) noise filter whose operating frequency exceeds several tens of megahertz. This paper proposes a novel and simple analytical method to calculate the winding stray capacitance of an HF three-phase common-mode (CM) inductor for the purpose of estimating its operating frequency range at the design stage. Results of measuring the CM impedance confirm that the proposed method can estimate the operating frequency range of an HF three-phase CM inductor while ensuring practically sufficient precision. Furthermore, the obtained results also show that the reduction in the number of turns does not always contribute to the enhancement of the operating frequency ranges of filter inductors.