I would argue that the credit of first 3D open-world game belongs firmly to Mercenary from 1985, and not to Hunter with it's 1991 release date.

Mercenary is a 3D vector environment where one can navigate the environment freely, on foot or by using different vehicles and aircraft, enter buildings and explore their interior. The graphics is much more limited than Hunter with the first Mercenary game from 1985 only using simple wire-frame graphics, you can most of the time just walk through.

Even with the technical limitations of it's time this game proved really captivating, once you made it past the initial hurdle of figuring out what the hell you're supposed to do in this game, after crash-landing on an alien planet in the introduction.

It was one of the few games that really fascinated me back then, and I spent many hours playing it.

There's a good introduction and walk-through of the game and it's sequels on YouTube: https://www.youtube.com/watch?v=YWTAEd0Z9uI