Triple monitor setup with PCI-Express and PCI slots?

Triple monitor setup with PCI-Express and PCI slots?

I currently have a PCI-Express video card which supports my dual monitor setup with 2 outputs. I want to add a third monitor via a PCI slot, which I already have a video card for. When I plug the video card into the PCI slot, it disables the PCI-E card. I can hear the sounds as Windows boots up, but get no video on any of the 3 monitors. When I unplug the PCI card, everything functions as normal.