You'd need something like this in your ServerLayout section for on of your X-servers in your xorg.conf. Obviously adjust the names and positions to suit your configuration. That being said, you cannot maximize to
N screens without having a desktop of that size.
Code:
Section "ServerLayout"
Identifier "XFree86 Configured"
Screen 0 "Screen0" 0 0
Screen "Screen 1" RightOf "Screen 2"
Screen "Screen 2" RightOf "Screen 0"
InputDevice "Mouse0" "CorePointer"
InputDevice "Keyboard0" "CoreKeyboard"
EndSection