Catalysis of the reaction: 3',4',5,7-tetrahydroxy-3-methoxyflavone + S-adenosyl-L-methionine(1+) = 3',4',5-trihydroxy-3,7-dimethoxyflavone + S-adenosyl-L-homocysteine.
3-methylquercetin 7-O-methyltransferase activity