Not sure whether it is an old wives tale or what but I'm sure I'd heard somewhere that women have something in their dna that means they absorb more alcohol than blokes and that's why they drink halves*.
If this were the case, her having 3 pints would be like you drinking 6, of strong gear and I bet in no time, so can't say I'm surprised.
*this used to be the case years ago, women on halves, although after about the third date, my missus went on the pints and hasn't drank a half since.
I sometimes feel I'd been duped