To begin a BO you have to always work 2 sts, so purl 2, then pass 1 over. That's 1 st bound off. Purl another and pass the first one over and you have 2 sts bound off. Purl one more and pass the first st over and that's 3 bound off. Count how many times you pass the first st over, not how many sts you purl (or knit).