It may be your adaptor arrangement. This Confused About 3 Screen setup on a Radeon 7700 thread describes a similar situation.
It seems that:
you need 1 native (DP) and 2 Legacy (HDMI or DVI) for the 3 screens to work.
The OP for that thread found a particular adapter for DP to VGA that worked, because it presented as a DP connection correctly.