You didn't list complete system details but I assume if you are running two gpu's at x16 you must have a CPU with 44 pcie lanes, you need to check your motherboard manual to see if you can add another pcie device without limiting your 2 GPU's, this may be motherboard specific is why more information is needed
I have two 1080s, and one samsung 2TB, but I want to run the 900P which of course take a PCI-E slot 😛 I want to know I can run all three PCI Cards at their full speeds, otherwise I wont give up an ounce of my GPUs x16s