What does the third eye of the Sage of the Six Paths do? The Sage of the Six Paths had Rinnegan in both in his eyes. He also awakened a third eye on his forehead. 



Is that a Sharingan of some kind? 

What powers does that third eye grant it's wielder?
From the Wiki


   After awakening his Rinnegan, he also manifested a red circle marking with two ripples patterns in the centre of his forehead, seemingly a byproduct of his ultimate dōjutsu manifested alongside his senjutsu training.


It references episode 426 of Naruto Shippuden and suggests it wasn't a true eye but rather a marking. There's no way to be sure if it gave him any special powers, but all 


   abilities he exhibited were in line with what we know was possible via his Rinnegan / Mangekyo / Senjutsu abilities so it doesn't seem to have given him anything. 


Additionally it's flat and lacks the depth that Kaguya's third eye had, also suggesting it was simply a marking. The best theory I can come up with is that it was an incomplete manifestation of Kaguya's third eye.