Final answer:
The phrase should be written as "three-mile" because it serves as a compound adjective before a noun and numbers less than ten should be spelled out in such cases.
Step-by-step explanation:
The correct way to write the phrase in question is "three-mile". When a number less than ten is used as part of a compound adjective (an adjective made up of more than one word) preceding the noun it modifies, it should be written in words, not numerals. In the sentence 'They arranged for a police escort along the three-mile route,' 'three-mile' is a compound adjective that describes the noun 'route.' Therefore, the spelled-out version 'three' should be used instead of the numeral '3'.