About the combat in Bg3, they want to keep it hidden "We want to show the combat, not talk about it"

They stated that they "With respect to the combat system, this is based on D&D, so we’re using their combat system" ( That is not the same as using the combat system of previous games)
https://www.pcgamesn.com/baldurs-gate-3/larian-developer-interview
but they tweak some things to make D&D a videogame experience. What things? We do not know, but they give us some clues:

Swen Vincke how fully his studio is adapting D&D’s character progression, and he said:

“That’s actually been one of the things that we’ve been struggling with, because it’s a very slow leveling process in the books,”

https://www.pcgamesn.com/baldurs-gate-3/leveling-system

Misses from dice rolls. “The very obvious one would be that you tend to miss a lot when you roll the dice, which is fine when you’re playing on the tabletop, but it’s not so cool when you’re playing a video game,” Vincke said. “We had to have solutions for that.”
https://www.pcgamesn.com/baldurs-gate-3/leveling-system

There is also another article that confirms the timeline of the games and the sinergy with WotC PA´s of Bg Murder in baldurs gate and the upcoming Descent into Avernus.
https://venturebeat.com/2019/06/06/...es-place-after-dds-descent-into-avernus/