I always figured 3PO was lying about his knowledge (or lack thereof) about the princess in the same way R2 lied about the restraining bolt interfering with the message, followed by "what message?" after the bolt's removal...

Based on what's given in the original film I guess I always assumed that the droids belonged to the captain of the Tantive IV ("our last master was Capt. Antilles...") but that Leia had sent for them (well, R2 anyway. That scene gives the impression that R2 knows exactly where he's going & 3PO is just tagging along). Perhaps R2 had a reputation for being a feisty droid that could get the job done where others failed?