Catalyzes the transfer of a methyl group from AdoMet to arsenite, producing methylated arsenicals. Involved in the conversion of As(III) to a number of methylated products. Reduces the arsenic toxicity in the cell and may contribute to the global arsenic cycling. [thioredoxin]-dithiol + arsenic triglutathione + 2 H2O + S-adenosyl-L-methionine = [thioredoxin]-disulfide + 3 glutathione + H(+) + methylarsonous acid + S-adenosyl-L-homocysteine 2 [thioredoxin]-dithiol + arsenic triglutathione + H2O + 2 S-adenosyl-L-methionine = 2 [thioredoxin]-disulfide + dimethylarsinous acid + 3 glutathione + 2 H(+) + 2 S-adenosyl-L-homocysteine 3 [thioredoxin]-dithiol + arsenic triglutathione + 3 S-adenosyl-L-methionine = 3 [thioredoxin]-disulfide + 3 glutathione + 3 H(+) + 3 S-adenosyl-L-homocysteine + trimethylarsine Highly dependent on the characteristics of the thiol cofactors used, with some of them (coenzyme M, homocysteine, and dithiothreitol) more efficient than GSH. Belongs to the methyltransferase superfamily. Arsenite methyltransferase family.