okay lets start from scratch.
You switched cables and no go. not a cable issue.
You put in a working bitmain controller and no go. not a bitmain controller issue.
So here is the issue the 1 board is dead is maybe
or the one board is mismatched and reads as dead. maybe
if it all worked fine for a while it is very likely the board died and is read as a mismatch.
bitmain firmware says fuck you all 3 run or we shut your ass down.
If you only want to run bitmain firmware if may mean you need to repair or reprogram the dead board.
But if you are willing to use after market products like braiins or epic both will allow you to run only 2 boards.
https://altairtech.io/product/epic-umc/. the board is not cheap but you never pay a fee for it once you hook it up
look at my gear all 3 boards are on. In your case you shut the dead on off and get 2 of 3 boards to work