Catalyzes the transfer of a methyl group from AdoMet to arsenite, producing methylated arsenicals. Involved in the conversion of As(III) to dimethylarsenate as the main product in the medium and also produces dimethylarsine and trimethylarsine gases. 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 Optimum pH is 7.5. Up-regulated by As(III). Deletion mutant cannot methylate arsenic and becomes more sensitive to As(III). Could be exploited in bioremediation of arsenic-contaminated environments. Belongs to the methyltransferase superfamily. Arsenite methyltransferase family.