Sooner or later is not now. They already said they would be recycling their D:OS2 engine for BG3.
An engine is a tool, you can do whatever you want with it. Aside from that, they have already said that they are not using the same version of the engine.
Baldur’s Gate III will not use the same game engine as the Original Sin series. “We’ve been in development on this game for several years already. It’s not the same engine as those two, but it is our own engine. It’s built on the technology that we already have,” Vincke said.
Why are people discussing something when they can't be bothered to google facts a few secs? I can tell you with absolute certainty that BG3 will be a third-person game with optional iso cam game like Neverwinter 2 or Dragon Age it's the next logical step in their development.