After checking risers and power connectors on each riser.
Next I would run at stock with no OC or power settings on all 12 cards. with and without MSIafterburner running.
If still an issue, I would check the BIOS and see if it is up to date. There was a memory issue back in 2016 with some 1070 cards. https://forums.evga.com/EVGA-GeForce-GTX-1070-BIOS-Update-v8604500070-m2565056.aspx
Go back to an earlier version of the miner and MSIafterburner, sometimes newer versions have bugs in them.
Next I would check the cards of oil seepage. I had one card that the pads they used to help prevent the overheating issues leaks oil and oil got onto the pci card pins and caused issues. I cleaned it per EVGA.
I would then check power usage of the miners. I had an issue once where an update for greatly improved performance but it drew twice as much wattage per card. could be that you don't have enough power to run all of them.
Try cards in different computer see if it causes issues.
Contact EVGA and ask for their help on a memory issue on one of your 3 cards in SLI mode erroring out.
RMA the card

Thanks for the advise. It will be a while before I get to dedicate some time to it. But will update if find the issue.