def METHOD_NAME(backend, union_subsets):
    (a, b, c), (da, db, dc) = union_subsets
    expr = a.union(b, distinct=True).union(c, distinct=False).order_by("id")
    result = expr.execute()
    expected = pd.concat(
        [pd.concat([da, db], axis=0).drop_duplicates("id"), dc], axis=0
    ).sort_values("id")
    backend.assert_frame_equal(result, expected)