When a time frame was first mentioned early this year, when Dragon Commander and the Divinity 3 (actual name not announced yet) were in pre-production, it was anticipated that both would be released late next year. As far as we know Dragon Commander is still on track for that time frame. Given the lack of details on Divinity 3 it is likely coming out after Dragon Commander, but we don't know if Larian will wait until after DC is out before ramping up promotions for it, if there is going to be some overlap of announcements, or if the release timing will end up being determined more by whether or not the development time matches the expected estimates for both games (just with the info out on DC now they have received requests that would require a fair bit of work to implement, for example).
From an older topic (search for posts by Lar_q with 'status' in the title for later updates, though there wouldn't be anything significant in them about Divinity 3):
There are lots of details out now for Dragon Commander, but not much about Project E (other than it having a top down camera view). See Lar's post
here for the first details of both projects, and there were some earlier comments in the older topic
Status update. The latest news on project E is in the more recent
Status update and
Status update (long overdue) topics. The most recent status updates of
June 9th,
June 23rd and
July 11th don't contain much info on the games (more behind the scenes stuff and other projects).
