Dear Gezoriugi, thank you for contacting us. Can you share information about your setup? Many cards support a triple screen (Eyefinity) setup but require (at least) one active adapter cable to make it work.
If you don’t use Eyefinity you should be able to get a triple screen setup working directly from the card. But the Acer monitor you mention seems to have a VGA (analog) input only. Since the 280X only features digital outputs you would need an active adapter to from either mDP to VGA or DVI-D to VGA.