select * from t1 join t2 join t3 on t2.a=t3.a on t1.a=t2.a;

create table t1 (a int); 

insert into t1 values (7), (2), (9);

create table t2 (a int);

insert into t2 values (5), (1), (7);

create table t3 (a int);

insert into t3 values (2), (7), (3); 

MariaDB [test]> select * from t1 join t2 join t3 on t2.a=t3.a on t1.a=t2.a;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'on t1.a=t2.a' at line 1

mysql> select * from t1 join t2 join t3 on t2.a=t3.a on t1.a=t2.a;

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

| a    | a    | a    |

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

|    7 |    7 |    7 |

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

select * from t1 join t2 left join t3 on t2.a=t3.a on t1.a=t2.a;