Not just 3 either, but multiples of 3. It even misleads some people into thinking they have to buy 3 for the "deal", since some other stores really do the "$x when you buy y" thing.
Over the last decade or so, I've seen stores really pushing buy one get one half-off, BOGOHO, which is of course just "25% off when you buy 2". It's riding the coattails of BOGO and overall is usually a pretty mediocre sale.
Edit: I've also seen BTGO cropping up, which is of course "33% off when you buy 3".