Added PCIe USB card, everything works as expected (3rd Rift sensor might need a cable shortening or other debugging). VR works well in quick test - good enough to warrant moving the rig to a case.

PCIe USB card here: https://www.amazon.com/gp/product/B01MQ5R7I1/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1