Ok, it took a bit, but I got it running on my Linux system (yea, I know it is a Windows app). This looks like a hack of an early Cura release.
As a test, I loaded the Raspberry Pi Rack case files (5 objects). I was able to select each object individually and move them independently. Are you sure you have 3 separate STL files and not 3 objects in 1 STL file?
Thank for the help GrueMaster, much appreciated. I don't see the behavior you mention (with the RPi rack files), I have prepared an example, see attached, to share.
So, using your example stl files, when I mouse over them, they highlight individually (as it appears in your 'example layout.png' file). I can select and move by holding the left mouse button and moving one object. I can also hold the right mouse button and change the camera view perspective (tilt & turn). Everything behaves similar to Cura.
Now granted, I am running the Creality Windows based Slicer in Wine on Linux and not on a true Windows OS, but that shouldn't make a difference in this case.