As far as footage goes, we've only seen 3PO mention Leia, but not actually encounter her. The very moment he walked in on her and R2, she jumped away behind a colon. I'm pretty sure someone as 'unimportant' as 3PO, at least on that particular consular ship, never got to see its consul. Therefore, he probably knew of the Princess, and her history, since it was common talk amongst the crew members, but never got to see her in person, which could perfectly explain why he did not know who the lady in the hologram was. Not to mention he got his memory wiped in ROTS.