Involved in the biosynthesis of benzylisoquinoline alkaloids. Catalyzes the 3'-hydroxylation of (S)-N-methylcoclaurine. (S)-N-methylcoclaurine + O2 + reduced [NADPH--hemoprotein reductase] = (S)-3'-hydroxy-N-methylcoclaurine + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] Alkaloid biosynthesis; (S)-reticuline biosynthesis; (S)-reticuline from (S)-norcoclaurine: step 3/4. Like CYP80B3b (AC I3V6B1), the full-length CYP80B3 is probably anchored to the membrane by a signal anchor. Localized to the sieve elements in the root and hypocotyl. Increases rapidly between 1 and 3 days after seed germination. Up-regulated upon elicitor treatment (at protein level). Belongs to the cytochrome P450 family.