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?
yes. the plugs will show up as little old "VGA monitors" on your display options. extend the desktop to these monitors. you should now have one BIG SCREEN (the one youre working with) and three LITTLE ones. again, make sure they're aktive.
i use GUIminer. in the DEVICE drop-down field, there should be something like
[0-0 Cayman]
[0-1 Cayman]
[0-2 Cayman]
[0-3 Cayman]
Start a worker for each of those. you should be getting ~310 mhash/s per card.
if you're command-lining, yes, the things should be usable from the commandline.