Catalysis of the reaction: N6,N6-dimethyl-L-lysyl9- + S-adenosyl-L-methionine = H+ + N6,N6,N6-trimethyl-L-lysyl9- + S-adenosyl-L-homocysteine. This reaction is the addition of a single methyl group to the dimethylated lysine residue at position 9 of histone H3, producing histone H3K9me3.
histone H3K9me2 methylase activity