The 3rd person who has it

[Assertn]

would kakashi have mangekyou then if he killed his best friend? apparently he doesnt though, since it wasnt able to counter itachi's mangekyou

either kakashi was holding back for no real reason against itachi, or he isnt the 3rd person

[Souryusen]

I suppose it's possible that Kakashi might have access to a gimped version of Mangekyo Sharingan... but I doubt Itachi would mention it. Perhaps there's someone else out there with Uchiha blood that isn't directly related to the main family. A bastard line perhaps?

[Eurasian]

definitely not kakashi. cuz well he's not an uchiha and that's been discussed.