From what I understand, you need an "Active Displayport to DVI Adapter". It plugs into a USB port to get power, plugs into your displayport socket on your ATI card, and then allows you to plug in your DVI monitor into the other end so your 3rd monitor will work.
Be warned. Many places are sold out of them, and the ones you find online will cost about $100.
I forgot to mention "Or a monitor that supports Displayport. From what I hear, only really expensive monitors have it, so might be best to just use 24" DVI monitors that you can get for $200 instead of shelling out $400-500 for a special "displayport" monitor lol.
one issue I've been having (have my 3008wfp connected to displayport) is that it will cut out on the displayport monitor for a couple seconds, like it's losing signal. I dont' know if people with the converters have th problem too.