I have 3 GPUs (one Vega 64, two RX 580s) and I can only consistently keep the vega 64 and one RX580 connected after reboots.  I've checked the risers (they appear to be good).  I saw on youtube that sometimes the video cards don't work without them thinking that they have a video card attached so I have two monitors (or I can use the HDMI plugin that fakes a monitor).  I have an older MOBO that should have the appropriate requirements but as I mentioned earlier, sometime I can get the rig to see the third card but sometimes I can't.