class A{
    @ManyToMany
    List<B> bs;
    //other attributes and getter setters
}

class B{

    @ManyToMany
    List<C> cs;
    //other attributes and getter setters
}

class C{
    //other attributes and getter setters
}

A:{
  "A_id": 1
  B:[
    "B_id": 1
    {
      C:[
          {"C_id": 1}, {"C_id": 2}
        ]
    },
    {
      "B_id": 2
      C:[
          {"C_id": 2}
        ]
     }
  ]
}

A_id    B_id    C_id
=====================
 1       1       1
 1       1       2
 1       2       2