1000w PSU is pretty borderline for mining with 6 card rig, to be honest (Always give yourself headroom with PSU. Efficiency is better and You can have a nice night sleep knowing that You aren't pushing Your PSU too hard)
Try GEN2 on Your PCI-E lanes.
That is very true!! and yes I tried Gen 2 a still problems with the 3rd GPU

I really don't know what else to try, I completed another fresh install of windows thinking it would be maybe a reg key that had got corrupt but yeah didn't work..
Debating to buy 3 more PCI risers and possibly RMA the motherboard for a new one or different one..
I've tried to reach out to Asrock but they haven't got back to me yet. Its so frustrating because this motherboard is meant to be a mining board, I wasn't expecting any of these problems...
I RMAd one of my H81 MOBOs on 20.09 (no POST) and I haven't heard anything about it from then... It seems that a lot of AsRock H81 Pro boards are faulty in some stupid way
Hope everything works out for You and You can get a new MOBO fast if this problem doesn't get fixed!