“He says it's nothing, sir. Merely a malfunction. Old data. Pay it no mind.”

3PO know that he is lying to Luke ^ when he says this, he saw Leia giving R2 the message a few hours before. Then as soon as Luke leaves the room (To go have some blue milk) he admonishes R2 for not trusting Luke. If he is saying to R2 "He's our new master, we can trust him" this demonstates he knew R2 wasn't telling the truth to Luke.

So I can't understand why people see a contradiction in the mention of the princess in private and then the denial later on in front of Luke (Who they just met).