Most 7850s would support it. Just look at the back for how many outputs it has. I have a Sapphire 7850 and I have 2 mini display ports, a DVI and an HDMI. The catch is at least one of the monitors HAS to be hooked up using display port for eyefinity to work. So you have to have a monitor that has a display port input or you need to buy an ACTIVE display port adapter.
Also... consider that a 7850 is a little weak for running games on 3 monitors. I imagine you would have to turn the settings way down to get playable frame rates.
EDIT: I was assuming you intended this for videogamesl. If you simply want to use 3 monitors you just hook them up to any 3 outputs if you have them and ignore what I said about the display port stuff.