It's from 《左传 郑伯克段于焉》. The meaning of the sentence is clear: (the measure of) the wall of big cities cannot exceed one third of (the wall of) the capital.
My question lies in the grammatical structure of 三国之一 (one third of the capital). I intend to think that 三 is temporarily used as a verb, meaning to divide into three parts. If my take on 三 is valid, are there any examples that can corroborate it?
But then, even if it holds, the Verb Phrase 三国 (dividing (the measure of the wall of ) the capital into three parts) doesn't connect smoothly with 之一, which naturally follows a Noun Phrase.