A.id A.name B.id B.name C.id C.name
-----------------------------------
1    Thing  2    Bee    NULL NULL
1    Thing  NULL NULL   1    Cow

(
  SELECT
    A.id AS a_id,
    A.name AS a_name,
    B.id AS b_id,
    B.name AS b_name,
    /* Substitute NULLs... for the other table C, for *all columns* */
    NULL AS c_id,
    NULL AS c_name,
    NULL AS other_c
    /* etc, other cols from C */
  FROM A
    LEFT JOIN B ON A.id = B.a_id
/* UNION, rather than UNION ALL  in case both tables have NULLs */
) UNION (
  SELECT
    A.id AS a_id,
    A.name AS a_name,
    /* This time substitute NULLs for B, again *all columns* */
    NULL AS b_id,
    NULL AS b_name,
    C.id AS c_id,
    C.name AS c_name
    C.other_c
  FROM A
    LEFT JOIN C ON A.id = C.a_id
)
ORDER BY
  a_id,
  /* sort the non-null B ahead... */
  CASE WHEN b_id IS NOT NULL THEN 0 ELSE 1 END