It was intended to be a trilogy, and Divinity 3 may well finish off the story arc with Lucian and Damian. However, there may (and should, IMO) still be additional games set in Rivellon. Any such games wouldn't necessarily continue to use the 'Divinity' name, but 'Divinity 4' is shorter than 'the RPG that Larian makes after Divinity 3 in the same game world'.
The Hitchhiker's Guide to the Galaxy was also a trilogy, but ended up with 5 books.
