Not a teacher!
Thanks for your replies. It seems I was correct to begin with (no hyphen needed between 'three' and 'mile').
The use of a hyphen does not clarify my example.
Maybe a look in Michael Swan's "Practical English Usage" will remove all doubt that a hyphen is required in your example:
"386 noun + noun (2): advanced points
5 measurement: a five-litre can
Noun + noun is used in measurements with a number before the first noun. The number is usually joined to the first noun by a hyphen (-)."
Some of the examples given: "a six-pound chicken". "a three-mile walk", "ten two-hour lessons".
Personally, I like
to do a run, but I don't think it is a good idea
to do a runner in because I don't want to have
to do a runner. - This has nothing to do with the original question, but I thought it is a nice example of how a slight change of words can totally change the meaning of an expression (from
running to
killing to
running away).
TomUK