So I was thinking about this some more, and it seems to me like it has to be either せえの or せいの, because that gives you three morae, which is similar in rythm to the English "1, 2, 3" with the fourth beat being when you start doing whatever it is you're about to do. At least it seems that way to me. I'd never thought about it before. I had always thought it was two units, but this makes more sense.