honk@honk ~/bin/Screenshots $ cat screen.sh 
#!/bin/sh
echo "Usage: screen.sh <filename>"

DISPLAY=":0.0" import -window root -display :0.0 tmp0.png
DISPLAY=":0.1" import -window root -display :0.1 tmp1.png
DISPLAY=":0.2" import -window root -display :0.2 tmp2.png

composite tmp1.png empty.png tmpA.png
composite -geometry +1024+0 tmp0.png tmpA.png tmpB.png
composite -geometry +2624+176 tmp2.png tmpB.png -quality 75 $1

rm tmp?.png