I recently upgraded to Ubuntu 20.04, and I normally work with three monitors. In a typical setup and installation once the displays have been setup in the order of preference the login screen would normally be displayed in screen 1. However, for some reason this was not the case in Ubuntu 20.04, and it would sometimes cause one of my monitors to stay on the bootsplach menu of my manufacturers motherboard. Nevertheless, here is a workaroud that helped.

• First, ensure that your displays are in the order that you desire by going to Settings > Devices > Displays and configure your monitors
• Next, copy monitors.xml file to gdm’s home folder.

sudo cp ~/.config/monitors.xml ~gdm/.config/monitors.xml

• Change the permissions on this new copied file.

sudo chown gdm:gdm ~gdm/.config/monitors.xml

• You must also uncomment the line below in “/etc/gdm3/custom.conf” .

WaylandEnable=false

Having made those changes, and after a reboot I now had my login screen on the desired display.