Gigabyte Z68XP-UD3 no more than two GPU's recognised

Hi,
I’m using an old PC to set up a mining rig, currently it’s working fine with a 3070 and 3080, I’ve got a pair of 5700XT’s to add but as soon as I add a third, it’s not recognised at all, only the other two cards. I’ve tried switching between slots, cards, risers and still no more than two cards.
I’ve had a look at the settings in the BIOS and can’t seem to see anything relevent. Issue is the same for windows or HiveOS

Does anyone have any suggestions? Would be greatly appreciated!

Enable 4g decode and gen 2 in pcie … in motherboard…