Mac Pro dark screen and cursor at startup

My Mac Pro won't startup. It begins Startup but stalls on a black screen. None of the fixes recommended work (I've tried them ALL). What I finally figured out is El Capitan doesn't like more than one monitor (I have three). When I unplug two monitors, it starts up fine. Yosemite worked fine with three screens, ElCapitan started fine if I unplugged one of the thee monitors. The latest version requires me to unplug two monitors so it can start up.