You said it yourself. You don't know those monitors are on separate video cards. As there are only 2, likely not. Still doesn't answer the question about... "multi-card extended desktop, especially when the VRAM is different" and perhaps "backwards compatibility". Nvidia NVS has made it a point for their cards to be backwards compatible. We all get it..."you like AMD".... but you don't know whether AMD has done that or not... and with OP's problem, that's at least a possible consideration. BTW... I never said "AMD doesn't make great video cards", you know.
D Current video drivers are 2 years old & card versions are almost 7-8 year old so I don't know if that will make any difference.
Process of elimination. The firmware of the card is very out of date. I always update my drivers and bios.
Power supply is brand new. Installed only 2 months ago. These cards use less than 25 watts. My power supply is 1350 watts.
M/C is 5 years old & have latest Bios available from Dell. I don't know how to update the firmware on each of these 3 cards so would appreciate help on that.
http://support.amd.com/en-us/kb-articles/pages/workstation-graphics-drivers.aspx Maybe the card does not have separate firmware but the latest driver came out in february, worth a try.
Not defending the brand but I did research on multi monitor systems & many vendors supplying M-M systems especially for traders used this particular card which AMD says is designed for Financial traders, My first 3 cards lasted for full 5 years without any problems running 12, 24 inch monitors.
T Tried that newest version & it gave problem with SnagIt app which I use extensively so went back to old Dec 2015 driver version.