Thank you for your suggestion. I too, had the yellow triangle until I followed your step:
"What got rid of the yellow triangle from the firmware in Device Manager was to be sure the battery was charged to 100%, and leaving the charger connected, I uninstalled
the offending yellow triangle firmware (deleting the driver)."
Deleting the driver option is what I hadn't done in my previous attempts to remove triangle.
Also, my next step differed from yours in that I simply restarted surface pro 3 after removing firmware and deleting driver.
It booted up perfectly and without the yellow triangle!
I have to say that the computer seemed to function perfectly even with the yellow triangle but I just didn't like seeing it.
The only issue I have left is with my surface pro 3 pen. It seems to work fine but does not show up in the list of bluetooth devices. Also, it doesn't open OneNote when
clicking the top of pen nor does it do a screen capture.
Any suggestions would be appreciated.