What gets me is, if Itachi had said that there were three people who were capable of using it, why would Sasuke say that he was the only survivor other than his brother? Afterall, he did tell Sasuke this the same night that he massacred the Uchiha. Maybe it's bad writing or maybe Sasuke just "forgot".

That's for the possibility of there being another Uchiha around; although it'd be nice if there were, I don't think it would fit in with what's already been written in the story too well. But then again, the twist they added onto the Mangekyou Sharingan wasn't going along with what's already happened either.