I must agree too Z170 chipset is having problem with 3+ cards on most boards, its prolly bios related, but who ever is doing testing first disable all non needed IO and internal devices as:
- sound
- ports
- any add in hw as firewire, wifi, etc
- set PCI-e to x1 mode if possible

And only then test it, also better use win10 because it should handle 8 GPU with no moded drivers