Catalyzes the transfer of a methyl group from AdoMet to trivalent arsenicals producing methylated and dimethylated arsenicals (PubMed:16407288, PubMed:25997655). It methylates arsenite to form methylarsonate, Me-AsO(3)H(2), which is reduced by methylarsonate reductase to methylarsonite, Me-As(OH)2 (PubMed:16407288, PubMed:25997655). Methylarsonite is also a substrate and it is converted into the much less toxic compound dimethylarsinate (cacodylate), Me(2)As(O)-OH (PubMed:16407288, PubMed:25997655). [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 An isoform missing exon 3 is produced with a premature stop codon at AA 23 and is targeted to nonsense-mediated mRNA decay (NMD). Devoid of methyltransferase activity. Belongs to the methyltransferase superfamily. Arsenite methyltransferase family.