BG3 is sat on a re-written Divinity 2 engine. (I guess call it Divinity 2.5 engine)
So hopefully there will be a similar toolset as DoS2. The DoS2 tools set was quite a high learning curve but very flexible.
They must be using an inhouse toolset to create levels / content etc. Its just what bits of the engine you want to expose to the community.
