Extension type instance inside CPython extension type

Question

I have the following extension types in C

//VECTOR 3
typedef struct
{
    PyObject_HEAD
    double x;
    double y;
    double z;
}Vec3Object;

static PyMemberDef Vec3Object_Members[]=
{
    {"x", T_DOUBLE, offsetof(Vec3Object, x), 0, "X component"},
    {"y", T_DOUBLE, offsetof(Vec3Object, y), 0, "Y component"},
    {"z", T_DOUBLE, offsetof(Vec3Object, z), 0, "Z component"},
    {NULL}
};

//TRANSFORMATIONS
typedef struct
{
    PyObject_HEAD
    Vec3Object position;
    Vec3Object rotation;
    Vec3Object scale;
}TransformObject;

//ENTITY
typedef struct
{
    PyObject_HEAD
    TransformObject transform;
}EntityObject;

And the following PyTypeObject(its also the same for the others and their respective members):

static PyTypeObject TransformObject_Type=
{
    PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "scene.Transform",
    .tp_doc = "Transform type",
    .tp_basicsize = sizeof (TransformObject),
    .tp_itemsize = 0,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
    .tp_new = PyType_GenericNew,
    .tp_members = TransformObject_Members,
};

Creating a Vec3 works fine but how would a PyMemberDef for TransformObject and EntityObject be defined?

I basically want to call the following Python code:

entity.transform.position.x = 12.5
entity.transform.rotation.y = -74.0

Or the following methods once in a PyMethodDef:

Vec3.Length(entity.transform.position)