Catalysis of the reaction: trihydroxyferuloyl spermidine + S-adenosyl-L-methionine = dihydroxyferuloyl-sinapoyl spermidine + S-adenosyl-L-homocysteine + H+.
N1,N5,N10-tris-(5-hydroxyferuloyl)spermidine O-methyltransferase activity