How? Please explain your reasoning. Maybe if it was only a fist fight but in the Naruto world the third stand no chance. Itachi is the worst type of match up for someone like him.
Itachi is a genjutsu master, the third raikage is only a nintaijutsu fighter. So that is one way Itachi can win easily. Regular genjutsu to make the third raikage lower his defenses or kill himself. Tsukuyomi is also there if he needs it.
Then Itachi has Susanoo with the legendary weapons, Totsuka sword and the Yata mirror. Now what is the only way that the third raikage attacks? Lightning enhanced nintaijutsu. And what does the yata mirror do? Block every conceivable element. So the attacks that worked against the eight tails would do literally nothing as the Yata mirror and Itachi's Susanoo are impenetrable.
And the Totsuka sword can seal the third easily, so another one in the win category for Itachi.
The only thing that the third raikage has on Itachi is strength and chakra levels (and most likely speed), which play no part in this battle since Itachi can just use genjutsu at the beginning of the battle, which is how Itachi starts off every battle anyway.