Either some people seriously lack Manga knowledge or just lack common sense.
The third Raikage is relative slow, usual Shinobi like Ao and those of the alliance could very well keep up with his speed and prepare various Jutsu, started with the rubber to huge walls of Doton, strengthened with Yoton, to protect themselves from the Sandaimes attacks. Do some guys want to tell me, that the Sandaime Raikage could escape from the great insight of the Sharingan and dodge Amaterasu, that arises at exactly the point Itachi aims for? And what about Tsukuyomi, that affects you instantly and leaves you mentally destroyed within a second? And what's about Susanoo, an ultimately strong, fast and huge samurai that protects its user from almost any attack, being able to cut through Kimimaros harder than steel bones easily and to tank a mountain busting Kirin without the Yata mirror? Even if Raikage could break through all stages of Susanoo, what certainly isn't the case, how the Sandaime will pass through the Yata mirror? Do you want to tell me, that Itachi will not be able to react fast enough, after the Alliance managed to build up a huge wall, in order to be protected against him? So after Sandaime hits Susanoo, whether with or without the Yata mirror, the Totsuka sword would slaughter him.
When it comes to the Mangekyo Sharingan, the Sandaime Raikage loses all his chances. Thats about it.