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As one of popular high-power power interface topologies between the energy storage devices and the dc link, the three-phase dual-active-bridge (3$\Phi$-DAB) dc–dc converter has received increasing attention. However, due to high number of semiconductor devices and multiple possible operation modes in the 3$\Phi$-DAB converters, the reliability and healthy operation become increasingly challenging compared with traditional single-phase DAB converters. This study presents detailed waveform analyses for 3$\Phi$-DAB converters under the normal and open-circuit faulty mode operation. Based on the analysis, a fast, cost-effective, reliable and sensor-less fault detection (FD) and fault isolation (FI) mechanism based on finite state machines for 3$\Phi$-DAB converters is presented in this article. Specifically, switch node voltages between the primary and secondary bridges of 3$\Phi$-DAB converters are adopted as the open-circuit fault (OCF) diagnostic signature and a low-cost OCF diagnostic circuit is utilized to perform the required signal processing. Using this technique, the open-circuit FD was achieved within 1/25th of the switching frequency and the open-circuit FI was performed within 1/6th of the switching frequency, which is the fastest FD and FI time for 3$\Phi$-DAB converters reported so far. Experimental results were acquired from a built prototype under various OCF scenarios to validate the effectiveness of the proposed technique.