Sorry to hear you are having issues. You can learn more about desktop pages by clicking on those instructions (Red box).
Just remember that your Desktop space is the entire 3 monitors you have. So the left most desktop border is on the left border on your left most monitor and your desktop right border is on your right border on your right most monitor.
And if you wish to switch desktop, make sure your mouse is on the desktop, make sure the desktop is selected. (click once on it) and than press the alt+left or alt right to switch pages.
Stardock Community Assistant.