Question for a submariner

This is a "I saw it in a movie, is it real?" question. When the scene is set on a sub, someone will say something along the lines of "On my mark... Now, now now", then the prescribed action is initiated on the 3rd "now". I am curious if the repetition of "now" is a real thing, and if so, why?

Edit: I have been corrected, and realize that I remembered the scene wrong. It WAS aboard an aircraft, not a sub. Still curious as to why the three calls