Gonk droids can be tortured to talk, and we see 3PO's willingness to surrender a time or two. It was just exposition to tell the kiddies: Remember Chewie and maybe Lando? Keep an eye open for them. Your daddy will nudge you when Lando raises his mask.
I agree. Threepio's a blabber-mouth. He'll tell you anything because he is scared of the consequences of not telling you. Why do you think they talked about wiping his memory at the end of ROTS? He knew Leia was a princess, but only because he was under the impression she was Bail Organa's real daughter. He didn't know any better. Protocol droids are known to crack under pressure. Astromechs do, too, but only if it involves keeping someone from dying. I suggest reading The Rise Of Darth Vader by James Luceno and The Han Solo Trilogy by A. C. Crispin. Those two books will explain 3PO's ignorance as well as an astromech's unwillingness to help unless threatened with a life or death situation, respectively. My point is this: They may have told 3PO that Chewie and Lando did not return as of yet in order to tell him a half truth... to throw Jabba or one of his minions off track. Artoo probably knows everything that's going to happen, but he is trustworthy enough that he won't go running his mouth in inappropriate situations. That's way he didn't get a memory wipe in ROTS.