Stream<Student> studentStream = Stream.of(
        new Student(1, 1, "A", 80),
        new Student(1, 2, "B", 90),
        new Student(1, 3, "C", 100),
        new Student(2, 1, "D", 90),
        new Student(2, 2, "E", 100),
        new Student(2, 3, "F", 80),
        new Student(3, 1, "G", 100),
        new Student(3, 2, "H", 80),
        new Student(3, 3, "I", 90)
);

Map<Integer, Map<Integer, Map<Integer, List<Student>>>> groupGC = studentStream.collect(
        Collectors.groupingBy(s->s.gradeId,
                Collectors.groupingBy(s->s.classId,
                        Collectors.groupingBy(s->s.score))

));

System.out.println(groupGC);

////////////////////////////////////////////////////////////////////////////////////
class Student {
    int gradeId;
    int classId;
    String name;
    int score;
    Student(int gradeId, int classId, String name, int score) {
        this.gradeId = gradeId;
        this.classId = classId;
        this.name = name;
        this.score = score;
    }
    @Override    public String toString() {
        return this.name;
    }
}