Mac Pro graphic board

I try to connect three screens with a Mac Pro. One has a resolution of 1680 x 1050 (connected via Minidisplayport + VGA adapter). The second has a resolution of 1080i and is connected directly on the DVI-port of the computer. The third one has a resolution of 1920 x 1080.

If the third monitor is connected to the Minidisplayport with a VGA-adapter, all three are being served. If this monitor is connected to the same port thorugh a DVI-adapter, the second monitor disappears from the list of the monitors.

Question: Is this behaviour caused by the graphic board? If yes, could it help to use the HDMI port of the monitor and connect with a HDMI-cable and adapter?

Thanks for helping.