Fire up Q3A and go to Setup/Controls/Look.
Set the 'Mouselook' option to your favorite button (You mentioned that was mouse 2).
Also, set the 'Free Look' option there to off.

That should give you the same setup you had in Q2. Although I suggest you try having mouselook always on. It frees up a mouse button and allows a quicker response to threats above and below your current position.