OS: Ubuntu 16.04
RAM: G.SKILL TridentZ 16GB (8GBx2) DDR4 3200MHz

Two GPUs work no matter how I connect them to motherboard, be it via risers or on 16x slots. If M.2 installed, only one GPU works. I'm currently using one 850W Thermaltake toughpower RGB PSU to run two GPUs, 850W should easily be able run three GPUs, right? Risers are SATA powered (instead of Molex).