You will need eather another graphics card, which will give you another 2 outputs or get a new HD 5000 series card which can support 3 outputs, but with a catch. (a Displayport adapter form a list of ones that are known to work)
What is your motherboard, how many PCIE slots dose it have?
What is your PSU, how many watts is it?
What do you plan to do with your 3 monitors? Gaming? nongaming?
Open CCC Advanced menu, click the Graphics button on the top left corner and select Desktops & Displays. Right click the big blue box in the middle of the window and select Create Group. Select the orientation of your monitors. Then one screen should go blue click which window that is so the computer can tell which monitor should be on the left/right or up/down.
Dell, Accell and Bizlink also have them (~$65-$100). Dont get the Apple one. I heard its hit and miss. Also, the display connected to this displayport should be the CENTER monitor. It makes web browsing etc easier to use/read.
Once connected, you need to set the displays via Catalyst Control Center. Use this as a guide:
It can be tricky to set up BUT once youre finished you wont be disappointed. Also, be prepared to turn down game settings to get playable frame rates (ex you may need to turn off AA). Remember the graphics card has to work 3 times harder to produce an image on your display!