This used to be completely configurable in the kscreen KCM, but it seems it disappeared somewhere along the way (maybe when I switched from X11 to Wayland?): the ability to select whether a monitor should extend the display area or mirror another monitor (and which one). Most of the time I do not really care, since I use only 2 monitors and they just make an extended display by default. I only use systemsettings to change relative positioning and fractional scale and voilà. But I also give a lecture in an amphitheater having 2 side-by-side video projectors and I would like the 2 projectors to display the same thing (as I used to do last year, but maybe under X11 plasma)... which seems to be impossible: kscreen decides that the third screen should just mirror my laptop integrated panel, with no apparent way to change this. Is this setting hidden somewhere or did this feature just disappear?
As a workaround, you can drag one screen on top of the other. You'll think you can't, but then it will snap to tne center when you drag it to just the right spot. *** This bug has been marked as a duplicate of bug 460433 ***
This workaround works provided a separate screen appears that you can drag around. My problem is that the screen I want to change the behavior of, only appears as an icon in the upper right corner of the representation of my laptop screen (which it is cloning). I cannot drag and drop and move this icon out of the laptop screen, for instance. So, so far there is no workaround I can think of.