Component of the PhsABC thiosulfate reductase that catalyzes the reduction of thiosulfate to sulfite and hydrogen sulfide, with menaquinol as the sole electron donor. Proton motive force (PMF) is required to drive transmembrane electron transfer within the reductase. The PhsB subunit transfers electrons between PhsC and PhsA. Binds 4 [4Fe-4S] clusters per subunit. Composed of three subunits: PhsA, PhsB and PhsC.