If Rockstar were to turn the saga of John Marston and Arthur Morgan into a trilogy, then story of Red Dead Redemption 2 and RDR to fit a story as large as Rockstar is known to tell. A RDR3 would likewise be unable to follow the original game, since the downfall of the Wild West in its totality is imminent - if it hasn't already occurred - by the time Jack Marston avenges his father and the credits roll.
Though Rockstar's games tend to emphasize exaggerated depictions of the United States, Red Dead is more historically focused, with both games having a heavy emphasis on the Old West itself. The downfall of Dutch van der Linde's gang from 1899 when RDR2 starts, to 1911 when John Marston is killed, mirrors the closing of the frontier in light of the rapid industrialization of the Wild West in the early 20th century, making another prequel Red Dead Redemption 3's best option.
There's very little room in the timeline for a RDR2 sequel to take place. The time between 1899 and RDR2's epilogue in 1907 is occupied by John attempting to start a new life for his family. Following the events of RDR2, Dutch is primarily laying low, managing to avoid the Pinkerton Detective Agency, and later the Bureau of Investigation, so thoroughly that John is coerced into finding him and the other remnants of the Van der Linde Gang in 1911. The gang is the major throughline of the Red Dead Redemption series, and its dissolution near the end of RDR2 and the subsequent death of its most notorious would make a sequel anywhere on that point feel disconnected.
The End Of The Old West Makes A RDR2 Sequel Near Impossible
The Red Dead Redemption series' strong Anti-Western themes position it as a depiction of the decline of the Old West told in reverse. In the first game, John is forced into helping the BOI track down the infamous Van der Linde Gang, a major component of a West Elizabeth gubernatorial candidate's promise to pacify the violent frontier and bring about a civilized and technologically advanced age. During the last year of John's life, automobiles are still rare in West Elizabeth, but real life 1914 (the same year as RDR's epilogue) would see the beginnings of the National Park to Park Highway, a project to make remote National Parks accessible to those able to travel by car.
In Red Dead Redemption 3 remains almost a certainty, but there's very little room for it to be anything other than another prequel.