use dummy plugs. they are easy as shit to build and cost ~0$ if you can get some 75ohm resistors (3 per plug). note: you need one plug per GPU so you might want to buy some DVI-VGA-Adapters.
google "vga dummy plug" for more info.
Sorry for the stupid question, but after I build them and plug them in will the devices automatically be assigned and visible to phoenix for me to run at the command line? or do I need to do something further?