They’re sort of the in universe audience/observers of the grand story, while being completely oblivious to the fact.
That has always been the point. But for some reasons 3PO got his memory wiped (I prefer to imagine the wipe didn’t work and that he still remembered a few things prior to ANH). The ROTJ “storytelling scene” stays a very charming moment in the saga.
3PO caméo does not work in R1. I think he should have been in the ending scene (instead of Leia), just for the fun of Vader seeing him 😃 (“Master Ani! Thanks the mak… oh… RUN R2 RUN !!!”).