My motherboard has
x2 USB 2.0
x2 USB 3.0
x1 USB 3.1
And I have an additional 2 USB 3.0 ports on the top of my PC case. (which I have reserved for a USB Hub and the Xbox One Adapter so it isn't behind my PC)
So I am thinking 2 sensors in the USB 3.0 ports and the Rift into the USB 3.1 port.