Depends on your motherboard, graphics card, and perhaps even OS.
Older discrete graphics cards only allowed you to use 2 out of the three or more connectors.
Modern cards let you attach monitors to all of them.
Some motherboards, particularly from oem builds have simple bios which do not allow integrated.discrete adapter flexibility. Modern intel chipsets allow three displays to be attached.
In the bios, you can set the adapter to be used for bios messages and startup. In windows, you can set the primary display.
Some older os versions allowed only one graphics driver. W7 and W8 allow many drivers.
What specifically are your parts?