In an interview with Gamesradar Larian's boss makes it clear that the new Divinity will not require any extensive prior knowledge of the universe; if you wish, the two Original Sin titles will suffice, as they will provide some context for the new adventure, both in terms of references and gameplay. Even though the earliest Divinity titles have their place in history, they are now very outdated.
"If you play the Divinity: Original Sin games, you will see things in this Divinity that reference those two. If you played the first Divinity, you will see it referenced properly inside of this Divinity. Ego Draconis, same story, or The Dragon Knight Saga, all of it is being referenced, but it's just part of the history of what happened in this world, and it helped shape the world to the point where it is now"
"If you really want to know everything, there are games that came before Original Sin. But they're a bit outdated by now, they are quite old"
In short: want the whole backstory? Play the Original Sin games. Just want an epic adventure in Rivellon without having to dig through two decades of gaming history? Don't worry. Larian lets you choose — thank goodness.
Are you planning to play through some of the old Divinity games before the new one comes out?