The Rock posts a lot of quotes from people on his Twitter page. I wouldn't say this is automatically an indication that he's hinting at Rock/Cena III.
If the second match didn't draw as much as the first one, then why assume a third match would do any better? If it were still the biggest match that Rock could be apart of, then it would be understandable, but it isn't. Rock/Lesnar (which has been hinted at on WWE television as well as through a tweet that Paul Heyman gave) is almost guaranteed to be a bigger draw than a third (or even second) Rock/Cena match. Rock and Cena already reached it's conclusion... Rock put over Cena (which is what Rock said he came back to the WWE to do) and even endorsed him at the end by raising his hand and such. Please let that be the end.