Yes finding 3 days alone is going to be a problem for most people. Also the dose required is large, someone survived 16g.
That's kind of true and not. I read from someone else here recovery from lethal doses is possible up to 16g, which I think is different to someone surviving 16g. It just means that if you don't take over 16g, it's possible for medical intervention to save you.
...But upon researching further I found something even worse, someone apparently surviving a Phenobarbital overdose of 18.9g with medical intervention.
We report affective utilization of continuous veno-venous hemofiltration for removal of phenobarbital in a patient who took twice amount of fatal dose and was hemodynamically stable. Historically treatment includes supportive care, activated charcoal and urinary alkalinisation along with the...
clinmedjournals.org
However, there are 2 factors that need to be taken into consideration:
1) He was found unconscious, but didn't specify how long he might have been unconscious for. It was possible he was found well under 3 days.
2) They suspected he had built up some tolerance to Phenobarbital due to long term use.