xdm problem

Hi everyone: I met a problem with the 3 screens linux system. when the system startups and logins X window by xdm/kdm automately, there are only two same screens working. However, when I start X by "startx" from console in runlevel 3, the 3 screens work well. I think there is some error in /etc/X11/xdm configuration file, but I couldn't find it. Could you give me some help? Regard Gao Jin