Catalyzes the transfer of a methyl group from AdoMet to arsenite, producing methylated arsenicals. Involved in the conversion of As(III) to monomethylarsenic and the conversion of monomethylarsenic to dimethylarsenic. 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.