IMO, If the person already has drinking problem, i.e., has drank a lot in the past, that could have affected the brain so just the 3 drinks could trigger a mental "snap" & a blackout. Also, if they drank on empty stomach, were taking meds, had low blood sugar, etc...all this can affect how they process alcohol if they've already got problem. Also, depending on how well you know this person, they could be sneeking alcohol so they had more in them than they're saying. I lived with a man like that who flipped out on me periodically but always insisted he hadn't drank 'that much' but after he died, I found liquor bottles stashed ALL OVER the place. Apparently he was drinking all the time and was able to function. He sucked on cinnamon candy all the time to cover up his breath.