It’s actually C-3PO, but what does that even mean? Nothing. It’s merely an alpha-numeric designation and therefore used as his name.
Even “Behind the Scenes” it doesn’t come from something as colorful as R2-D2’s name. That was a code given during the filmming of American Graffitti signifying the sound code for “Reel 2; Dialogue Track 2” of shooting. It was spoken simply as “R2-D2” and George Lucas adopted it as a catchy droid designation.
He has offered no similar story for the inspiration for “C-3PO,” so either he just came up with a combination he liked, or just never felt the story behind the name was worth telling.
