Yes, Madara and Izuna read the tablet with their sharingans at first, learned about the mangekyo sharingan and gained that. Then they could read more and probably learned about the rinnegan then.
Now, Sasuke only read the tablet using his sharingan. As far as I know, he never came back to Konoha after he got his mangekyo. So he hasn't read the tablet for years now. The only way he could learn of the rinnegan without the tablet would have been from Tobi or Orochimaru. But I doubt Orochimaru talked about it, cause Sasuke would have easily been interested in it.