What version of Bios are running?
For the History only Z68 Intel boards can utilize the onboard controler with a dedicated graphics card. All other boards just can't, even if it is stated in their documentation.
In order to use 3 monitors you need an Ati or Nvidia card that support Triple Display at the same time. I have a Board like yours with an ati HD5570 connected to 3 monitors utilizing 1DP,1 DVI and 1 VGA connection.