It really comes down to what you write in your log book. A to B is X ctry, any flight that leaves the airfield and goes somewhere is X-ctry.
The 3 mile definition in the ANO is to set limits on the privileges of an AFI or FI(R) in that they are not allowed to authorise a student on their first solo cross country i.e. a flight more than 3nm from the airfield. It has no other meaning beyond that. At some airfields circuits are bigger than that!