Rob,
Thanks - excellent suggestions. It turned out I think you were right:
server #1 actually has a bad DIMM (good to know)
servers 2 and 3 only have 1 PS plugged in..
which leads me to my next question - how did I not get alerts on these conditions vs just a red health X? Misconfiguration?
They are a mix of Win 2012 and 2008 if that matters.
Thanks again.
Damian