Well, I remember there WAS that image when Itachi was talking about what it takes to make the Eternal Magekyou to Sasuke and there was the image of what kinda looked like a Susano'o with four Sharingans.
But I really think the Rinnegan being his is just a lie, or at least, a creative interpretation(like the interpretation that Madara was the former Mizukage). I mean, he's an Uchiha, so his Sharingans are probably real.
I'm just wondering if his mask has 3 eyes now because, after losing one of his Sharingans, and obtaining two Rinnegans, if he didn't have both Rinnegans implanted, one in his old Sharingan's place, the other in his forehead.