I have the same problem, but these forums are not going to find help.
There is no solution.
AMD just lie in their advertising.
According to AMD specifications:
http://www.amd.com/us/products/desktop/graphics/r9/Pages/amd-radeon-hd-r9-series.aspx # 5
They should be able to use 3 monitors WITHOUT using DP port.
4 Monitors using the DP port.
And 6 with a MST hub.
I've tried three different versions of drivers on my windows 8 and no way to use more than 2 monitors. As much as I ask in the forum nobody gives a solution (probably because there is not).
If you have purchased this card with the idea to use 3 or 4 monitors (like me), you've been scammed ( like me )