Yep...in case anyone comes back to this thread, I purchase an AMD Sapphire Radeon FLEX HD 6770 1GB DDR5 PCIe 2.1, and Unity works perfectly across all three now

Yep...in case anyone comes back to this thread, I purchase an AMD Sapphire Radeon FLEX HD 6770 1GB DDR5 PCIe 2.1, and Unity works perfectly across all three now using the proprietary drivers, and setting the "Multi-display desktop" option for each monitor in the amdcccle utility.