Add shortcut/launcher arg: -subwindow. Edit %LOCALAPPDATA%\Sniper3\Settings.ini Resolution_Width & Resolution_Height (no border *). ShiftWindow (+ border = borderless).

(PLP screenshot with bezel effect)
NOTE * Deprecated: tester used "resolution+border" values in ini, which is not strictly proper.
View guide for general help with the instructions.
File Attachments:Sniper Elite 3 2014.jpg