The JAA don't say anything about cross country flights other than that for certain licensing requirements, you must have so many hours cross country.
The legal definition of 3 nm from the arerodrome is with respect to the privileges of the AFI and FI(R) ratings, with regard to solo cross country authorisation.
For a cross country flight to be counted for licensing purposes it must either go from A to B where the distance is such that some navigational skills are exercised i.e in excess of 15 minutes crusing flight time would be an absolute minimum. Alternatively, it should be a route with defined turning points. CAP53/54 clearly stated that all turning points on a navex should be logged.
There is no legal definition, but what you do, must be reasonable if it is to be accepted.