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 di- and trimethylated species, with trimethylarsine as the end product. 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 Belongs to the methyltransferase superfamily. Arsenite methyltransferase family.