For Beyond Compare use BComp.exe

By default when setting Beyond Compare as the merge tool BCompare.exe is executed.  BComp.exe is the proper exe to use on windows for merging.  BCompare doesn't work for merging if a beyond compare window is already open since it will use that window and then exit immediately.

See: https://www.scootersoftware.com/v4help/index.html?command_line_reference.html

"BCompare.exe - This is the main application. Only one copy will run at a time, regardless of how many windows you have open. If you launch a second copy it will tell the existing copy to start a comparison and exit immediately.

BComp.exe - This is a Win32 GUI program. If launched from a version control system, it should work just fine. If launched from a console window, the console (or batch file) will not wait for it."