Three GPUs
A third dual-slot GPU can be installed by removing the dual 10Gb ethernet card and moving the AMD GPU to its slot. The third dual-slot GPU can then take up that perviously AMD GPU occupied slot.
Y-Cable
The machine has two 6-pin PCIe power cables which can be used to power the THRID GPU using a "Dual 6-pin Female to Single 8-pin Male PCIe Power Y-cable".
Image below shows how the power from two 6-pin 75W connectors are combined to deliver 150W to one 8-pin connector, "ground" is connected to the second sense pin (green) of the 8-pin connector.
As shown below, the 3rd Nvidia GPU can reduce the air-flow to the AMD GPU substantially, so the AMD W5100 GPU must not be running any heavy load to keep its temperature low.
