Try reinstalling your driver
Done that many times without any change. Was on the 15.9 drivers, now on 15.10 betas in the hope that the problem was addressed.
Sent from Samsung tablet
Drastic action: Borrowed a mate's spare Nvidia GTX960 card, realise it is not as powerful as the R9 290 but........it works with multi monitors!!!!
Swapped out the R9, plugged in the 960, 3 x DP/vga cables and bingo all 3 monitors returned to life - no "input not supported" or black screens.
So come on AMD get the problem with your drivers and Win 10 sorted, I have been a long time supporter of yours and regret going over to Nvidia but I need the PC to work properly which you are not providing at this moment.
Will keep an eye on the driver updates to see if there is any indication that this anomaly has been fixed and I can re-install the R9 (my mate hopes that is not too far off!!!)