Catalysis of the reaction: L-lysyl79- + 3 S-adenosyl-L-methionine = 3 H+ + N6,N6,N6-trimethyl-L-lysyl79- + 3 S-adenosyl-L-homocysteine. This reaction is the successive addition of up to three methyl group to the lysine residue at position 79 of the histone H3 protein, producing H3K79me3.
histone lysine N-trimethyltransferase activity (H3-K79 specific)