Catalysis of the reaction: S-adenosyl-L-methionine + 3,5,7,3',4'-pentahydroxyflavone = S-adenosyl-L-homocysteine + 3-methoxy-5,7,3',4'-tetrahydroxy-flavone.
S-adenosyl-L-methionine:3,5,7,3\',4\'-pentahydroxyflavone 3-O-methyltransferase activity