my 2018 mac mini has 4 thunderbolt/usb-c slots but only 2 displays actually work at a time.

I have a 2018 mac mini with 4 thunderbolt/usb-c slots. I have 3 displays and the first 2 of the 3 displays plugged into the thunderbolt/usb-c slots will work and it doesn't matter which slot any of the displays are plugged into. but plugging in the 3rd display does not work. If I unplug one of the displays though, then then this 3d non-working display starts working. And replugging in the unplugged display does not make the display work until one of the other displays is unplugged.

Is there a limit? Do I have to configure something? Why have 4 slots if only 2 can work?