C-3PO is often written as it is pronounced: See-Threepio
In The Phantom Menace it was revealed that C-3PO was built by Anakin on Tatooine from scrap and spare parts salvaged from junk yards. For the whole of Episode 1 he lacks the outer casing with which we (having already seen him A New Hope) associate with him.
In A New Hope, when he gets dragged into the plot by R2-D2, he is serving as a diplomatic protocol droid aboard Princess Leia's starship, Tantive IV. I hadn't noticed until someone pointed it out to me that C-3PO's right lower leg (i.e. from the knee down) is silver, in contrast with his otherwise golden-coloured body. An interesting fact. It serves as a reminder of his background as an unprofessionally-made droid.