The Third Mangekyou Sharingan User - Who is it?
Okay check this out... I think that the third mangekyou sharingan user is none other then Kakashi, Obito was a true Uchiha, as stupid and narutoish as he was. His eye should be capable of mangekyou. On top of that, why would Gai train to not look into Kakashi's eyes if not for the mangekyou? There would be no point.
Kakashi has never fought to the point in any of the manga battles to have to use his mangekyou. Zabuza was a formidable opponent but when Kakashi got serious, he fell pretty easily. Orochimaru and Kakashi's brief skirmish means nothing, besides maybe Orochimaru doesn't know that Kakashi has the mange.
I dunno, just a thought...