It seems that Red Dead Redemption 3 is indeed in development. Possible references to the game were found in the source code of Grand Theft Auto 5, which leaked on December 25th. No details of the unannounced project, other than its name, were found in the leaked files. Recently, there has been increasing talk about Red Dead Redemption 3. In September, insider MyTimeToShineHello stated that the third installment is already in development. Additionally, in early December, actor Roger Clark, who played Arthur Morgan, expressed confidence that players will eventually see a new Red Dead.
If the information is accurate, then the project is likely in the early stages of development, as all of Rockstar Games' efforts are currently focused on Grand Theft Auto 6, expected to be released in 2025.