Well, this is more or less what you want to do
If you want it to check regardless if you click, you have to put the left mouse event into the tick event
What this BP does:
Saves screen size in the begin play. When Left mouse is clicked, divides screen size by 3 and checks in which of the 3 fractions of the screen the mouse is positioned.
Obviously you have to swap my “guns” with your own 
Good luck!
