WHY CAN I ONLY USE 2 OF MY 3 DISPLAYS!???????

Hi, I recently ordered 2 more displays for my PC and when I plug in 1 DVI plugs=s into the graphics card and one HDMI, only the middle one shows up, when I unplug 1 of the corner monitors, I can use the other two fine. WHY DOES THIS HAPPEN!????????????????? (I have checked its not a hardware issue, I can use all three, just not at the same time)

Thanks,
- Dan