Ace said:
tobi and kakashi are the third considering that tobi is obito or at least using his body. tobi has half the sharingan and kakashi has the other half.

i think it is pretty obvious that the 3rd MS user is Madara aka Tobi. Not Obito/Kakashi. Madara is the founder of the Uchiha and perhaps the greatest Sharingan user.

i have argued about this already in a previous post, as well as many others. but i quote the relevant part of it here.

melo;12458703 said:
WHO IS THE THIRD MS USER THAT ITACHI IS REFERRING TO?

In chap.225p04, Itachi says to Sasuke, "if you open your eyes to the truth... including myself, there will be three people who can handle the mangekyou sharingan."

NOT KAKASHI. Yeah, Kakashi did have his version of the Mangekyou, but it seems Itachi was not referring to him as the 3rd guy. He was in fact surprised when Kakashi knew about the adverse effects of Mangekyou use... and was daring enough to face off again with his Mangekyu. and THEN Kakashi showed him his sharingan (we did not see what Itachi saw). But by the looks of it, he seemed to have gotten it that Kakashi has gained the Mangekyou. he was dumbfounded and shocked, in ch257p9-10, Itachi says, "Kakashi-san... impossible, you..."

See also ch364p04-05, when Kisame remarked that Itachi appears to be "crying in the rain" because of the death of Sasuke and him now being the sole member of the Uchiha clan. Itachi replies, "no... he's not dead yet... and besides... the storm... has passed." The pauses may indicate his knowledge of someone else that Kisame must not know. (Tobi=Madara?)

Being the originator/founder of the Uchihas and everything that is said about Madara's power, the Mangekyu should be a default for him.

Basing from http://www.onemanga.com/Naruto/364/18/ Tobi did claim that he is Madara, as the best sense of "the sharingan's true power, my power, Uchiha Madara's power..."

what is the true power of the Sharingan but the Mangekyu?

in chapter 142 page 13-15:



Itachi said to Kakashi: "It's impressive that without any Uchiha blood relation... you can use the sharingan to that extent... however... your blood isn't like ours... so that eye doesn't suit you... i'll let you experience sharingan... or our clan's true power...!!!"

"We are the true masters of the Sharingan..."
and shows off his MS.

[Kakashi exclaims in his mind: "It couldn't be!" Kakashi was able to return the favor in their next encounter. Itachi was now the one surprised that Kakashi got a version of the MS.]

"Some resistance could be made to this Mange-Sharingan. However this Sharingan has a special eye jutsu. Only a Sharingan user with the same blood as me can defeat me. My illusion jutsu Tsukiyomi is unbeatable..."

And then Itachi demonstrates by casting the Tsukiyomi on Kakashi, which only an MS can do.

see http://www.onemanga.com/Naruto/142/15/
;P :sgan

In other words, Itachi knows that Madara still exists and was referring to him as the person who has the blood and the capacity to have the true power of the Sharingan, the Mangekyu.

We just don't know yet if Itachi indeed knows Tobi is in fact Madara.