How are you hooking them up. Do all monitors have Display Port and HDMI? If not, and all are HDMI or DVI only, you will need to hook two up HDMI or DVI. The other will need an ACTIVE Display Port to HDMI or DVI adaptor.
I have the 570 with one hdmi and 3 displayports. It will run any two of the outputs but not three. I have active displayport to hdmi converters on the two displayport outputs. I have restarted, updated drivers, unplugged / replugged, etc. Only two (any two) will come up.