# register table:
+------+---------+
| rid  | owner_id |
+------+---------+
|    1 |       3 |
|    2 |       5 |
|    3 |       6 |
|    4 |       6 |
+------+---------+

# user table;
+-----+------------+
| uid | name       |
+-----+------------+
|   3 | jim        |
|   4 | jack       |
|   5 | johnnie    |
|   6 | james      |
|  10 | susi       |
|  11 | sandra     |
|  12 | debora     |
|  13 | christine  |
|  14 | michelle   |
+-----+------------+

# lookup table
+------+------+----+
| luid | uid  |rid |
+------+------+----+
|    1 |   10 |  1 |
|    2 |   12 |  1 |
|    3 |   13 |  1 |
|    4 |   14 |  1 |
|    5 |   10 |  2 |
|    6 |   14 |  2 |
+------+------+----+

# lookup table structure:
+------+---------+------+-----+---------+----------------+
|Field | Type    | Null | Key | Default | Extra          |
+------+---------+------+-----+---------+----------------+
| luid | int(11) | NO   | PRI | NULL    | auto_increment |
| uid  | int(11) | NO   | MUL | NULL    |                |
| rid  | int(11) | NO   | MUL | NULL    |                |
+------+---------+------+-----+---------+----------------+


mysql> SELECT register.*,user.*,COUNT(lookup.uid) AS usercount
FROM register LEFT JOIN user ON register.owner_id = user.uid
LEFT JOIN lookup ON register.rid = lookup.rid
GROUP BY lookup.rid;

expected result would be:
+------+----------+------+----------+-----------+
|  rid | owner_id | uid  | name     | usercount |
+------+----------+------+----------+-----------+
|    1 |        3 |    3 |  jim     |         4 |
+------+----------+------+----------+-----------+
|    2 |        5 |    5 |  johnnie |         2 |
+------+----------+------+----------+-----------+
|    3 |        6 |    6 |  james   |         0 |
+------+----------+------+----------+-----------+
|    4 |        6 |    6 |  james   |         0 |
+------+----------+------+----------+-----------+