Comment # 21 on bug 1206863 from Stefan Dirsch

Honestly such a complex setup, i.e. 3 GPUs running with one Xserver would be
too complex for me. Intel + nVidia GPU with suse-prime, ok. But Intel + 2
nvidia GPUs? Maybe I would try to disable at least one of the GPUs by a
hand-crafted xorg.conf, but at least I wouldn't expect this to work out-of-the
box. OTOH you have it running with your Dell 5520 with a comparable hardware
setup out-of-the-box.

But maybe things would be easier if you could at least disable one of the GPU
in your firmware. Could be that one of them is exclusively connected to your
internal display, so that on you can't disable of course.