Installed 3x R9 280X, two are mining fine, one <30kh/s. Please help

All three cards are MSI Twin Frozr R9 280X. Seasonic Platinum 1kW PSU. Gigabyte GA-F2A88X mobo. One GPU is plugged directly into the mobo in the primary PCI-E 16x slot. The other two are using the 1x USB 3.0 based riser cables (both powered). Everything boots up fine and CGminer loads without issue, but each time I load it, only two of the cards actually mine at full speed (about 710kh/s). The other one is working, but extremely slowly - about 20-30kh/s. What's odd is that every time I load it up, it's a different card that's not functioning at full speed.

I suspected it could be some setting in BIOS to enable the lanes in the PCI-E cards, but I read through the entire manual and there's nothing about it.

I don't have dummy resistor plugs, could this be the cause? Only reason I didn't suspect this to be the issue is that sometimes the card that's at low speed is the only one that actually IS plugged into the monitor. The other two are functioning fine.

Any thoughts? Help is really appreciated.