Either a USB to VGA device or a second inexpensive video card. If you only have 1 pei-e slot, there are PCI and PCI 1 cards available but they're rather old tech for the most part. If you do get a second card go with Nvidia so you don't have driver issues. Although in my opinion the best solution would be a Flex edition Saphire card, no DP or DP adapters required for 3x.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102960