flatMapTuple3<L, R, R2, R3>  function 
 
        
Either<L, Tuple3<R, R2, R3> >  Function(Either<L, dynamic>  either)
flatMapTuple3<L, R, R2, R3>( 
    
- Either<L, R3> f(- dynamic a
 
A variant of flatMap that appends the result to a tuple.
Implementation
Either<L, Tuple3<R, R2, R3>> Function(Either<L, Tuple2<R, R2>> either)
    flatMapTuple3<L, R, R2, R3>(
  Either<L, R3> Function(Tuple2<R, R2> a) f,
) =>
        fold(left, (t) => f(t).p(map((r3) => tuple3(t.first, t.second, r3))));