Only run off the primary card.
280x have 3 ports - DVI-I (plus DVI-D), HDMI and Display Port. So there's enough ports. If you're running 3 with game on then you will need help from crossfire. Its max 2560 x 1600 on one.
Can I only run off primary card?
I am planning on gaming with these in CF . But the monitors(Acer H226hql) do not use a display port. I know I could run an active adapter from the primary cards DisplayPort to DVI and plug that into the monitor, but instead can I just occupy an extra DVI from the second card while using the two primary DVI connections available.