Catalysis of the reaction: S-adenosyl-L-methionine + a 3-demethylubiquinol = S-adenosyl-L-homocysteine + a ubiquinol.
S-adenosyl-L-methionine:2-octaprenyl-3-methyl-5-hydroxy-6-methoxy-1,4-benzoquinone-O-methyltransferase