你可以试着用union

SELECT columnA FROM table1
    union
    SELECT columnB FROM table2
    union 
    SELECT columnC FROM table3

如果只需要不同的值，可以使用union

SELECT columnA , 'T1' t_name
FROM table1
UNION
SELECT columnB, 'T2'  
FROM table2
UNION
SELECT columnC , 'T3' 
FROM table3

但是如果你需要所有的值，你应该使用union all

SELECT columnA, 'T1' t_name
FROM table1
UNION ALL
SELECT columnB, 'T2' 
FROM table2
UNION ALL
SELECT columnC , 'T3' 
FROM table3

UNION ALL 或者 UNION 帮你。取决于是否需要复制：

SELECT columnA FROM table1
UNION ALL
SELECT columnB FROM table2
UNION ALL
SELECT columnC FROM table3

欲了解更多信息，请阅读：https://dev.mysql.com/doc/refman/8.0/en/union.html
编辑：
您可以使用第二列和固定文本来区分应用程序中的记录：

SELECT columnA,'TABLE1' FROM table1
UNION ALL
SELECT columnB,'TABLE2' FROM table2
UNION ALL
SELECT columnC,'TABLE3' FROM table3

如果您采用并集的思想并添加一个行号，那么您可以创建一个行号列表，然后使用该列表左键联接到三个表。例如

drop table if exists t1,t2,t3;
create table t1(id int);
create table t2(id int);
create table t3(id int);

insert into t1 values(1),(3);
insert into t2 values(10),(20),(40);
insert into t3 values(3);

select s.rownumber,t1.id t1id
        ,t2.id t2id
        ,t3.id t3id
from
(
select @r1:=@r1+1 rownumber from t1 cross join(select @r1:=0) r1
union 
select @r2:=@r2+1 rownumber from t2 cross join(select @r2:=0) r2
union 
select @r2:=@r3+1 rownumber from t3 cross join(select @r3:=0) r3
) s
left join (
select id,@r4:=@r4+1 rownumber from t1 cross join(select @r4:=0) r4
) t1 on t1.rownumber = s.rownumber
left join (
select id,@r5:=@r5+1 rownumber from t2 cross join(select @r5:=0) r5
) t2 on t2.rownumber = s.rownumber
left join (
select id,@r6:=@r6+1 rownumber from t3 cross join(select @r6:=0) r6
) t3 on t3.rownumber = s.rownumber
;

+-----------+------+------+------+
| rownumber | t1id | t2id | t3id |
+-----------+------+------+------+
|         1 |    1 |   10 |    3 |
|         2 |    3 |   20 | NULL |
|         3 | NULL |   40 | NULL |
+-----------+------+------+------+
3 rows in set (0.00 sec)