A linkedIn profile for a Rockstar Employee showed that he had been working on Red Dead Redemption 3 for at least last 2 years. Meaning there was work started on the new game right after the Red Dead Redemption 2 was released. Now this could be a typo as well. But thinking about it people hardly ever make typos in their LinkedIn profiles.
Now the profile mentions work being done in the AI department so this could very well be true that Rockstar is working on the AI for the next game engine already so that they can polish some more features in when the next game is actually being developed and worked on.