Am I correct in thinking that you have 2 x FirPro v5900?
What type of connection to the 3 monitors?
Solom, I've been running dual monitor on Win 7 Pro for years now and never had a problem with them being detected by the OS. They are extended not mirrored. Each connected by an independent DVI cable. They are older Acer 1680 X 1050. Hope you get them figured out. If so, you might like to check out a program called DisplayFusion [ DisplayFusion: Multiple Monitors Made Easy! • Binary Fortress Software ] which will allow you to run panoramic photos across them, which is something, unlike Win 8 and above, Win 7 won't do out of the box. And it just might help you configure them on your system. It's basically free for general use -- unless you want the advanced features and then it's like $25 for a lifetime upgrade.
I did a little research on your system. It has onboard graphics. The VGA connector on the motherboard. When you insert a graphics card into the PCIE 16 slot, your motherboard detects it and turns off the onboard graphics. So if you have a monitor connected to the motherboard VGA connector it will not work with a GPU in a slot . And only the two monitors connected to your GPU will work. Unless the third monitor, as the FirePro V5900 supports three monitors, is connected to the GPU via a connector on it as it has a Dual-Link DVI and two 1.2 Display Ports. If your motherboard has 2 PCIE X 16 slots, you can run a FirePro v5900 GPU in each slot. But the FirePro v5900 can't be run in SLI or AMD CrossfireX as there's no way to connect them with a bridge.
These links might help you: