CREATE TABLE t1 (i1 int, c1 varchar(20), pk int) engine=myisam;

CREATE TABLE t2 (pk int, c1 varchar(20), i1 int) engine=myisam;

INSERT INTO t2 VALUES (1,'a',NULL),(2,'a',777),(3,'a',192),(4,'a',284),(5,'a',2),(6,'a',NULL),(7,'a',-912),(8,'a',5),(9,'a',NULL),(10,'a',-184);

CREATE TABLE t3 (pk int, c1 varchar(20), i1 int) engine=myisam ;

INSERT INTO t3 VALUES (1,'a',-145),(2,'a',6),(3,'a',1),(4,'a',NULL),(5,'a',NULL),(6,'a',NULL),(7,'a',NULL),(8,'a',889),(9,'a',146),(10,'a',177),(11,'a',NULL),(12,'a',-183),(13,'a',-137),(14,'a',NULL),(15,'a',NULL),(16,'a',-433),(17,'a',NULL),(18,'a',2),(19,'a',3),(20,'a',5),(21,'a',-484),(22,'a',369),(23,'a',-192),(24,'a',-163),(25,'a',5),(26,'a',NULL),(27,'a',NULL),(28,'a',1),(29,'a',NULL),(30,'a',NULL),(31,'a',127),(32,'a',-424),(33,'a',3),(34,'a',-144),(35,'a',5),(36,'a',913),(37,'a',NULL),(38,'a',6),(39,'a',-173),(40,'a',4),(41,'a',101),(42,'a',-795),(43,'a',201),(44,'a',103),(45,'a',NULL),(46,'a',NULL),(47,'a',6),(48,'a',NULL),(49,'a',9),(50,'a',NULL);

SELECT count(t3.c1) 

FROM t3 LEFT JOIN t1 LEFT JOIN t2 ON t1.i1 = t2.i1 ON t3.i1 = t1.i1

WHERE t2.pk < 13 OR t3.i1 IS NULL;

MariaDB [test]> SELECT count(t3.c1) 

    -> FROM t3 LEFT JOIN t1 LEFT JOIN t2 ON t1.i1 = t2.i1 ON t3.i1 = t1.i1

    -> WHERE t2.pk < 13 OR t3.i1 IS NULL;

+--------------+

| count(t3.c1) |

+--------------+

|           21 |

+--------------+

1 row in set (0.003 sec)

MariaDB [test]> SELECT t3.*

    -> FROM t3 LEFT JOIN t1 LEFT JOIN t2 ON t1.i1 = t2.i1 ON t3.i1 = t1.i1

    -> WHERE t2.pk < 13 OR t3.i1 IS NULL;

+------+------+------+

| pk   | c1   | i1   |

+------+------+------+

|    4 | a    | NULL |

|    5 | a    | NULL |

|    6 | a    | NULL |

|    7 | a    | NULL |

|   11 | a    | NULL |

|   14 | a    | NULL |

|   15 | a    | NULL |

|   18 | a    |    2 |

|   20 | a    |    5 |

|   25 | a    |    5 |

|   17 | a    | NULL |

|   26 | a    | NULL |

|   27 | a    | NULL |

|   29 | a    | NULL |

|   30 | a    | NULL |

|   35 | a    |    5 |

|   37 | a    | NULL |

|   45 | a    | NULL |

|   46 | a    | NULL |

|   48 | a    | NULL |

|   50 | a    | NULL |

+------+------+------+

21 rows in set (0.005 sec)

MariaDB [test]> SELECT count(t3.c1) 

    -> FROM t3

    -> LEFT JOIN t1 LEFT JOIN t2 ON t1.i1 = t2.i1 ON t3.i1 = t1.i1

    -> WHERE t2.pk < 13 OR t3.i1 IS NULL;

+--------------+

| count(t3.c1) |

+--------------+

|           17 |

+--------------+

1 row in set (0.001 sec)