For a third match to work Cena would obviously need to win at this Wrestlemania. The crowd, for the most part won't be happy with that outcome and I would think that Vince would want to hold off on the rubber match until Mania or Summerslam.
I could easily see Cena beating the Rock fair and square at Mania, have Brock Lesnar run in afterwards and attack Cena to prompt the Rock to re-enter the ring and help Cena out. That way, Cena and Rock close the show together to keep the crowd happy, Extreme Rules is set as a 3-way match for the title or as Rock & Cena vs Brock & Punk and we have an easy set up for Rock vs Brock at either Summerslam or Wrestlemania 30, with Rock vs Cena 3 being at the other of those two events. If I had to call it I'd say we'll see Rock vs Cena again at next year's Wrestlemania.
I could easily see Cena beating the Rock fair and square at Mania, have Brock Lesnar run in afterwards and attack Cena to prompt the Rock to re-enter the ring and help Cena out. That way, Cena and Rock close the show together to keep the crowd happy, Extreme Rules is set as a 3-way match for the title or as Rock & Cena vs Brock & Punk and we have an easy set up for Rock vs Brock at either Summerslam or Wrestlemania 30, with Rock vs Cena 3 being at the other of those two events. If I had to call it I'd say we'll see Rock vs Cena again at next year's Wrestlemania.