create table ten(a int);
insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table one_k(a int);
insert into one_k select A.a + B.a* 10 + C.a * 100 from ten A, ten B, ten C;

create table t0 (a int, b int, c int);
insert into t0 select a,a,a from ten;

create table t1 (a int, b int, c int);
insert into t1 select a,a,a from ten;

create table t2 (pk int, b int, c int, primary key(pk));
insert into t2 select a,a,a from one_k;

create table t3 (pk int, b int, c int, primary key(pk));
insert into t3 select a,a,a from one_k;

explain select * from t0 
where t0.a in (
  select t1.a 
  from t1 
   left join t2 on t2.pk=t1.b
   left join t3 on t3.pk=t1.b
  );

+------+-------------+-------+--------+---------------+---------+---------+----------+------+------------------------------------------------------------------+
| id   | select_type | table | type   | possible_keys | key     | key_len | ref      | rows | Extra                                                            |
+------+-------------+-------+--------+---------------+---------+---------+----------+------+------------------------------------------------------------------+
|    1 | PRIMARY     | t0    | ALL    | NULL          | NULL    | NULL    | NULL     |   10 |                                                                  |
|    1 | PRIMARY     | t1    | ALL    | NULL          | NULL    | NULL    | NULL     |   10 | Using where; Start temporary; Using join buffer (flat, BNL join) |
|    1 | PRIMARY     | t2    | eq_ref | PRIMARY       | PRIMARY | 4       | j22.t1.b |    1 | Using where; Using index                                         |
|    1 | PRIMARY     | t3    | eq_ref | PRIMARY       | PRIMARY | 4       | j22.t1.b |    1 | Using where; Using index; End temporary                          |
+------+-------------+-------+--------+---------------+---------+---------+----------+------+------------------------------------------------------------------+

set optimizer_switch='semijoin=off';
explain ...

+------+--------------------+-------+------+---------------+------+---------+------+------+-------------+
| id   | select_type        | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+--------------------+-------+------+---------------+------+---------+------+------+-------------+
|    1 | PRIMARY            | t0    | ALL  | NULL          | NULL | NULL    | NULL |   10 | Using where |
|    2 | DEPENDENT SUBQUERY | t1    | ALL  | NULL          | NULL | NULL    | NULL |   10 | Using where |
+------+--------------------+-------+------+---------------+------+---------+------+------+-------------+