It was nice to see C-3PO show up for the Prequel Trilogy, but the odds for him to be that important to the prequels (as he was in the original trilogy) is pretty staggering high.
Sure enough, George Lucas made it where he was created by a very young and enslaved Anakin Skywalker.
His ownership goes through many hands just like his future, one of the first places he ends up is the home of the Lars family.
I did think it was hilarious that he became (accidentally) a bot for the Trade Federation Droid Army.
During Revenge of the Sith he serves mostly as an attendant for Senator Padme Amidala.
And he was there to witness the birth of Luke and Leia. Then he gets his mind wiped and spends about another 20 years wandering around until Luke Skywalker miraculously runs into him again.