For the third monitor you have to have an ACTIVE adapter, i dont know what you use, but an example:
say that you have HDMI, DP, AND DVI.
the first two screens you can use DVI HDMI OR DP depends on how many connections the card have.
But for the 3'rd screen, you will need an active adapter, for example an DP to DVI/-orHDMI active adapter.
you cannot run eyefinity (three monitors) without an active adapter. It's just the way it is.