Find some dead HDTV motherboards (from the cheap "untested / not working / sold for parts" category sales, not proper replacement parts). Most of them will have 3 HDMI ports (some might have as much as 5, some might only have a single one but that's rare) which is enough for 3 "rigs" (it's sufficient to have 1 display connected to 1 of the cards per motherboards with Windows>7, I prefer Win10).