If a cell is over discharged it can fail shorted, this is why 2 wires measure the same voltage,
Was the pack balance charged before use? if not then 1 cell could have been marginal while the other 2 where over charged.
If it was, and you discharged the pack to the point it fell out of the air, that cell was likely the weakest in the pack and collapsed from ~2.7V to 0 in a few seconds (there is almost no capacity at this end) Its not uncommon for the cells to be slightly mismatched, but in general if one cell is out by 20% from the others i get a little suss,
A easy way to spot this is how long it takes to balance charge a pack, if they all come out of the box at a similar charge, ideally it should be seconds to balance in, If its 10 minutes, it likely means you have either 1 strong or 1 weak cell confusing things.