The card is able to output only one VGA signal, there's your reason. Rest of the signals must be digital: DVI + HDMI or DVI + HDMI-DVI adapter.
Ok. Since I am not the most educated on graphics cards, are you saying that since I am already using the 1 - VGA port, that I cannot use a DVI to VGA adapter on the DVI port to convert the signal? How would the card know or recognize that the signal is being converted?