@Cookbot "I thought I'd save the £3,000 buying..." the cheapest, most basic (in hardware and firmware) conversion boards...
Sorry it hasn't worked out.
12v 5A is only 60 watts, and is at the bottom of the lower limit of a PSU for a 27" 5K monitor.
Test you failing boards and screens with a better spec PSU, because if the one you are using isn't giving the full 60 watts then that could be the cause...
To trouble shoot what is at fault - AA1 or screen panel or cable - you have to test each one systematically.
You have one good AA1, connected with one good cable to one good panel.
So you need to test, and write down the results, of using:
The good AA1 and panel with the other cables.
The good AA1 and the good cable to the other two panels.
The 'bad' AA1s, one at a time, with a good cable to a good panel.
Then report the results here and we may be able to help. 😉