Catalysis of the reaction: S-adenosyl-L-methionine(1+) + trans-aconitate = (E)-2-(methoxycarbonylmethyl)but-2-enedioate + S-adenosyl-L-homocysteine.
S-adenosyl-L-methionine:(E)-prop-1-ene-1,2,3-tricarboxylate 3\'-O-methyltransferase activity