This enzyme catalyzes three distinct methylation reactions for converting phosphatidylethanolamine to phosphatidylcholine. a 1,2-diacyl-sn-glycero-3-phosphoethanolamine + S-adenosyl-L-methionine = 1,2-diacyl-sn-glycero-3-phospho-N-methylethanolamine + H(+) + S-adenosyl-L-homocysteine Phospholipid metabolism; phosphatidylcholine biosynthesis.