From the pictures of the drive along with what has been reported the following conclusions can be made. There was an error in the amount and/or type of voltage applied to the 3 drives. This resulted in all 3 of the drives failing in the same manner. The catastrophic failure only seen in the third drive is easily explained.

The third drive was the weakest link in the chain. For whatever reason, and there are numerous possibilities, the destroyed area of the PCB of that drive provided the least resistance to ground. This is why there is the amount of damage that is seen. Once this happened there was a direct or close to a direct short to ground. This protected the other drives from any further damage and is also the reason for the power supply shutting down.

The position of this drive on the cabling is coincidence. This outcome would have most likely been the same regardless of the position of the drive along the cable. However, there is a remote possibility that had it been further up the cable any drives after it may have survived.