Actually, the phrase was an old saw in radio copywriting, where you repeated things three times. The formula does have its applications in technical writing, but it would take a stretch to make it fit. For one, if we wrote everything three times, our instructions would balloon accordingly. Yes, we should give the reader a clue as to what we're going to tell them, and it's a good idea to give a summary at the end. But how many people have the time or interest to wade through all that?