HELP! Can't connect more then 2 GPU's

[Scofield]

Im using a Asrock P81 Pro BTC mobo, along with some MSI r9 390 gaming 8gb cards.

When I plug 1 or 2 cards in, it works fine. But when I add a 3rd card, I don't get any singal to my monitor. Sometimes it boots, I can tell from the led's on my keyboard. It seems like a power issue to me, but I'm not sure why. I've tried many diffrent GPU's, risers and PCI slots..

I use 1x Corsair 650w to power the mobo and 1 GPU. I use another 1000w PSU to power the other two cards.

[klondike_bar_recovery]

powered risers should be powered by same PSU as the attached GPU.
non-powered risers draw power from the main PSU, so the connected GPU must also use that main PSU
otherwise, if the PSUs share a 12v or GND signal together it can cause lots of issues.

assuming your power setup is fine - its likely your bios. make sure that the PCIe is set to gen1, or possibly gen2 (not gen 3 or auto)