counting down

"Okay, a count of three: one .. two .. three .. cheese! .. ." Well, the click on five is exaggurated, but go on three, or the count after three?

Just count down: it's easy, everybody understands it exactly, the time "zero" is aligned at an interesting moment (the event, not some randome point, 3-4s before the event). And it also helps cut down on confusion in critical rock-paper-scissors matches.