Catalyzes the reduction of sulfite to sulfide. This is the terminal oxidation reaction in sulfate respiration, a process catalyzed by the sulfate-reducing bacteria. [DsrC protein]-disulfide + 2 A + 3 H2O + hydrogen sulfide = [DsrC protein]-dithiol + 2 AH2 + H(+) + sulfite [DsrC protein]-S-sulfanyl-L-cysteine/L-cysteine + 3 A + 3 H2O = [DsrC protein]-disulfide + 3 AH2 + 2 H(+) + sulfite Binds 2 [4Fe-4S] clusters per subunit. Binds 1 siroheme per subunit. Heterohexamer of two alpha, two beta and two gamma subunits.