I just purchased a HD 5850 video card with the displayport and eyefinity technology. My question is will I be able to do the three monitor display running xp 64bit?
Yes you should be able to.
Note: in order to have all your monitors connected to the card you need to have at least 1 monitor that has a display port connection. If you don't you will need to buy an active display port adapter, seeing as only the DVI connections+the display port connection can be used for Eyefinity with 3 monitors.