On hisdigital.com it say it require
PCI Express® based PC is required with one X16 lane graphics slot with 2 slots space available on the motherboard
So it's need 3 slots?
Oh, what it is referring to is that it needs a free PCI-E slot on your motherboard. It also is a double-width graphics card, requiring the slot the gpu is plugged into and the slot below it be unoccupied.