I have used two types -- this type I've had in operation the longest:

https://www.amazon.com/gp/product/B073W9KCFC/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

I use the bottom PCI-E slot for a video card -- just like the 4 card systems -- and the top two slots for one splitter each (two cards per splitter) -- and the third slot down has the riser plugged right into it.

I can get a few pics if you need.