UHD 620 three external monitors

I own a Dell XPS 9370 running Fedora 30 with Gnome. It has an integrated UHD 620, details can be found on the Intel website: Support for Intel® UHD Graphics 620.

Due to the Intel site it supports up to three monitors. Setups with the internal screen and two external monitors work perfectly fine. But at home I have three external monitors. So I would like to disable the internal screen and use all the external monitors. In theory that should be three monitors and the integrated graphics card should be able to handle that.

Anyway if I connect the third external monitor (it does not matter which of them), it stays black. Also xrandr just returns the internal monitor and the two connected ones. Disabling the internal monitor xrandr --output eDP-1 --off does not help, xrandr still sees only two displays. All monitors are connected using USB-C ports.

Does anybody have an idea how to disable the internal monitor and have three external ones instead? Is this even possible?