He is only doing Episode II, but last I read he was going to introduce Threepio is Owen's droid who nobody wants because he is so eccentric. So Anakin will not have made him, but he will end up still being Padme's droid by Ep. III. The idea is that whoever makes the Ep. I edit that precedes SOTDS will remove him completely so that the introduction in SOTDS will be the first time we meet him.