On starting the game I noticed I cannot use the mouse to scroll or select anything in the setup/config/options. The mouse works in gave but only for looking, zoom and firing. tried changing mouse settings, restarted game, reinstalled game etc ...
Can't see mention of it in forum or on internet.

EDIT.

Its not the mouse.
After testing various settings Arma 3 got minimized. When maximized again, it opened on a second monitor (I have 4)

Revised issue:

When Arma 3 is on main (Primary) monitor (HDMI) of a 4 monitor setup, mouse will not work as stated above. It will operate properly on 2nd 3rd and 4th monitor.