According to an interview with George Lucas in TheVulture we learn that Jar Jar was largely inspired by Disney's Goofy;
"I can't even begin to tell you how much of an influence Disney has
had on me," Lucas told fans at Disney’s D23 expo in Anaheim on Friday.
"I will say one secret that nobody knows: Not many people realize that
Goofy was the inspiration for Jar Jar Binks."
Whereas the major influences behind C-3P0 are generally acknowledged to be evenly split between Akira Kurosawa’s The Hidden Fortress and Fritz Lang’s Metropolis with a healthy dose of McQuarrie's amazing concept art sketches and Anthony Daniels' own input into the character.
Q. C-3PO was original written as sleazy used-car salesman-type of
character, but on-screen he was more nervous than anything else. What
brought that to the character?:
Daniels: That was George's idea... he was always [sleazy] in the script, but
it's how you interpret [the character]. That's the difference between
actors. George never told me he wanted a sleazy guy. So I just looked
at it. I had six months working with the script before we started
filming because we were making the costumes, so I just interpreted it
and George didn't have time to correct me. I'm afraid 3PO stayed the
way he was. He is 3PO, he isn't me. He is this strange creation that
came out of somewhere