So where will the third one take place? That question is closely tied to the story. If, like the relationship between the first and second games, the third is a prequel or a spin-off, it is quite possible the current map will once again be part of a new, larger map, meaning the old locations would still be accessible. That would imply a map of insane size, which would certainly suit a GTA but not necessarily an RDR. Who wants to spend 40 minutes riding from one end of the map to the other?
Another possibility is that the story moves forward chronologically, putting us at the end of the first game in the shoes of Jack Marston. In that case, maybe the link with the previous games stops there, and Jack, after killing Edgar Ross, is forced to flee to keep his former associates from finding him. The problem with this theory is that by the time we step into Jack's shoes, we are already in 1914. To put that in perspective, that is two years after the Titanic sank. We drift a bit too far from RDR's western world, and it is hard to picture Jack going off to war in Europe too, which would be completely off-topic.
So, a new map? A map merely expanded like in the second game? All bets are open. In the meantime, check out a huge map created by a fan.