Eyefinity needs a displayport output for anything more than 2 monitors. There are 2 types of DP to DVI/HDMI adapters, active and passive. You need an active adapter. Within the active adapter area, there are 2 more types: Single Link and Dual Link. Single Link will handle up to 1080p (or maybe 1920x1200) and is around $30-40. Dual Link handles up to 2560x1600 and is $130.
SLI "surround vision" works differently. Whereas CF you can only use outputs on the top card, with SLI you can use the outputs on both cards. Each card can support 2 outputs, so you can plug 2 monitors into the top card's DVI/HDMI outputs and 1 into the bottom card (assuming they have those outputs).