So, this is an ongoing issue which I thought would get sorted out after 14.x.
I have 13.251 installed, and works fine. All three screens work perfectly, no issues. Upgrade to any later version, and I must pick two screens. Attempt to enable the third? Pick one to disable. How come the new drivers are limiting? Surely the AMD devs can write this back in?
First two pics are on 15.7. Last 3 pics are 13.251.