Reduces chlorate and bromate. AH2 + chlorate = A + chlorite + H2O Binds 1 [4Fe-4S] cluster. Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Optimum pH is 7.5. Heterotrimer of alpha, beta and gamma subunits.