Mitochondrial protein-lysine N-methyltransferase that trimethylates ATP synthase subunit C. Trimethylation is required for proper incorporation of the C subunit into the ATP synthase complex and mitochondrial respiration. L-lysyl-[protein] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl-[protein] + 3 S-adenosyl-L-homocysteine Belongs to the ANT/ATPSC lysine N-methyltransferase family.