This took some trial and error on Google search, since a lot of disciplines have things they call "the rule of three". But this must be it:
"The mastery of the `Rule of Three' whereby one multiplied the first and second terms and divided the product by the third was considered a milestone in calculating. The `Double Rule of Three' included the terms -- "inverse, transpose, direct proportion" -- all Greek to less than an advanced student."
This comes from the description of contents of an 1830s school textbook.
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