I have 3 Viewsonic VA2231wm monitors connected via DVI. I have 2 AMD Radeon HD5700 adapters. Device driver is 15.201.1151.1008.
I ran the AMD autodetect utility and it says I have the latest driver. The Windows driver update utility says I have the latest driver.
All 3 monitors worked just fine prior to upgrade to Windows 10. Now Windows does not detect the 3rd monitor.
The HD 5700 can support a maximum of two passive display connections DVI/HDMI/VGA. A third display must use a native displayport to displayport connection, or a displayport to a DVI active adapter.
What adapter are you using? Does it have the word ACTIVE written on it?