Methylates myricetin and dihydromyricetin at 2 sites. Inactive towards 16-hydroxytabersonine, the phenylpropanoids 5-hydroxyferulate, caffeate and their CoA-esters, flavones and flavanones possessing 2 or 3 B-ring hydroxyl groups. S-adenosyl-L-methionine + a 3'-hydroxyflavonoid = S-adenosyl-L-homocysteine + a 3'-methoxyflavonoid. S-adenosyl-L-methionine + a 5'-hydroxy-3'-methoxyflavonoid = S-adenosyl-L-homocysteine + a 3',5'-dimethoxyflavonoid. The N-terminus is blocked. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.