Without debating the ethics/morality/philosophy of the issue, let's note that C3PO is not a sentient being according to the laws of the galaxy, and therefore can not be a slave or free "person".
C3PO is merely, like any droid, a piece of property, legality-wise.
Practice-wise, there were some autonomous droids (e.g. IG-88, or Prince Xisor's bodyguard) but they were this way for practical reason - nobody (successfully) claimed ownership of them once they've gone autonomous (and they were not that way originally).
Linked Wikia article's on legal status states (without giving sources, so not sure how canonical the info is):
Droids were typically treated as property. More advanced droids sometimes received proportionally more respect. Some protested the status of droids as slavery. This was a contentious issue, owing in part to the difficulty of assessing levels of artificial intelligence, and determining which droids if any could be considered properly sentient. Although emancipation of droids was a radical concept, some opposed memory wipes except for very simple droids.
The Naboo considered higher level droids equal as fellow sentients. Under the Galactic Empire, no laws protected droids from abuse and those who showed them respect were looked upon as strange.
As far as droids rebelling, it did happen at least once. From http://starwars.wikia.com/wiki/Great_Droid_Revolution :
The Great Droid Revolution was an uprising of droids on Coruscant which occurred in 4,015 BBY and the first conflict of the Old Sith Wars.