Counting to three is a common way to indicate to kids that you're being serious and they need to do what you're saying, and I find it helpful with my kids. It's associated with Thomas Phelan's 1995 book 123 Magic, but it's older than that:
Tom Chapin's 1988 song Nick Of Time includes a variant.
My parents used it with me, probably also around 1988.
My dad says he learned it from my aunt, who says she used it with her kids in the mid 1970s. She doesn't remember where she learned it.
There's something about it that feels reasonably recent to me, where it would feel out of place in a book from 100 years ago? But does anyone know more about its history?
(It's hard to search for uses, since it is often just called "counting" or "counting to three." I wrote to Phelan and Chapin without a response.)