It's possible that that third one has the PCIe power issue many people are experiencing with some reference rx 480 cards.
This seems very unlikely because the last thing that would be damaged would be the onboard audio in this scenario. Your PCIe slot is still fully functional (I assume).
You should know that those boards very frequently come with problems. I RMA'd an Asus x79 saberooth about 4 times in a row. The first time, the board's capacitors leaked and ruined all of my components. It isn't uncommon for an x79 motherboard to overvolt and damage one of the components. On one occasion, I had a dimm slot die on the sabertooth.
You can read reviews online (like on newegg) and see for yourself.
Oh yeah one more thing.
Don't RMA the board. Asus has terrible RMA and they will not send you a new one! Each one they've sent me comes with its own set of problems.
They will send you a used board or a refurbished one.
If no other problem occurs with the mobo, I suggest simply buying a sound card.
Show your pic or nothing happens
he likely forgot to plug in the extra power for the pci-e slots that you need to run 3way crossfire on that board.