You don't really go 3d but it can contain 3d objects.Like Insert 3D draw procedures and provide a texture for the object.
A Nice way done is the Game Maker. It actually does not support 3D , rooms are flat as in BYOND but it can contain 3D objects. You can create a First Person Shooting Game and lots..
I recommend to try Game Maker, cause it can extremely give ideas that can be added to BYOND.Though the Defect with Game Maker is that
- it does not provide an active HUB/Website for the Games..(the reason BYOND is cool).
- the procedure_names are a paragraph long
- etc....
Since I do not know anything about 'BYOND' Development(I believe it was created in 'C'/'Java'), So I really cannot say if BYOND gods can add this but you ought to take a look at Game Maker.
And that prevents you from using the forum search, which would have linked you to replies (by these 'BYOND gods') to similar suggestions?