Who told you that? I am a fan of Divinity games, but i dont want BG 3 to be D:OS 3. These are completely different games, even if they are both RPG titles.
Same. Even within the Divinity series I'd like to see Divinity 3 become a thing but I haven't been disappointed with the excursion into DOS and DOS2 in spite of them being very different games; as a newcomer to BG I'd expect BG3 to be BG, not Divinity wearing BG's clothes.
I could argue that IMHO DOS2 was something a diversion away from the Divinity that I'm familiar with and I could gripe about that, and have done a bit, but not to the point of saying "this is unacceptable". Similarly, I wouldn't be surprised if BG3 may diverge somewhat from its predecessors but I think that's necessary in order to make a new game a new game: there's little to be gained in reinventing the wheel, after all. I'm reminded of the futility of what was IME a particularly notorious example which was the seemingly endless carping about Oblivion not being Morrowind II, and while it wasn't perfect, it certainly wasn't
that bad (and was very, very good with the addition of some overhaul mods: though that's a matter of taste and some preferred theirs vanilla) and its corollary, Morrowind wasn't as astonishingly awesome as Oblivion's detractors claimed. Just ask any Daggerfall fan.

Anyway, the point of my ramble is that games evolve. Forcing them to stay too "true" to the original is as bad as making too many changes.