I think it was a famous magazine editor. I must be butchering the quote because Google search is absolutely no help on this one. In particular, I really don't remember how the third rule was exactly phrased.
I remember reading about a famous editor at a famous magazine that was known for edgy content. I think it probably existed in the 1980s or earlier. The editor was known for accepting submissions covering a very wide variety of topics, and they had very high standards for quality, and they had these three rules. I wanted to look them up to remind myself of exactly how they went.