How to enable third monitor connected to onboard intel graphics


I have a NVIDIA Corporation GF119 [GeForce GT 520] in my system which supports two monitors. That’s what I have been using for a long time but my mainboard also has a DVI & a VGA connector to which I now hooked up a third monitor. After I activated the onboard graphics in the BIOS, Manjaro does recognize it now: mhwd-l shows the following section:

> 0000:00:02.0 (0380:8086:0102) Display controller Intel Corporation:
                  NAME               VERSION          FREEDRIVER           TYPE
           video-linux            2018.05.04                true            PCI

but the third screen does not show up in Plasma System settings.
How can I add it to make it work?