i'm not doubting the three gallons from a medical standpoint just that i don't know of any camelbacks that carry that much. Most of the big ones are 3 litres so i was guessing.
And three litres shouldn't be enough to kill you but it's still impressive. (Ever try to drink 3 litres of water fast?) Point is most of us start gagging if we drink that fast (it's a protective mechanism.) That's why i wonder if something was wrong that made the guy unnaturally thirsty or maybe if his serum sodium was really low to start with.