Yeah, three packs together is supposed to be OK if you only do it once in a while. You're not supposed to do any more than that, but I remember reading a thread here where doctors had actually recommended it to people with really heavy periods. But yeah, for most people it's fine so long as it's not a regular occurance. The only problem is that your period is heavier afterwards (or I'd guess it was, it was when I skipped on period) and there is an increased risk of spotting (didn't have that problem with running two packs back to back, but never tried 3)