map3<R>  method 
Maps a parsed Record to R using the provided callback, see
MapParserExtension.map for details.
Implementation
@useResult
Parser<R> map3<R>(R Function(T1, T2, T3) callback,
        {bool hasSideEffects = false}) =>
    map((record) => record.map(callback), hasSideEffects: hasSideEffects);