I am having the same issue, only I have a laptop and two monitors. I usually have the laptop docked and unopened since it only supports two monitors at a time. When it boots, it uses my primary monitor for grub and the iniitial splash screen, but then insists on using the (closed) laptop monitor to display the login screen. I then have to open the lid to see the login form. Once I login though, it switches back to the external monitors. I would like a way to force it to use the external monitors is the lid is closed. Is there a setting for this?