O-methyltransferase involved in the biosynthesis of the permethylated L-rhamnose moiety of elloramycin, an antitumor polyketide. Mediates the methylation of the hydroxy groups at the 3'-position after the sugar moiety has been attached to the aglycon. 8-demethyl-8-(2-O-methyl-alpha-L-rhamnosyl)-tetracenomycin C + S-adenosyl-L-methionine = 8-demethyl-8-(2,3-di-O-methyl-alpha-L-rhamnosyl)-tetracenomycin C + H(+) + S-adenosyl-L-homocysteine Antibiotic biosynthesis. Belongs to the methyltransferase OleY/MycE family.