load=# create table t1 ( id integer, name varchar(32) );
CREATE TABLE
load=# create table t2 ( id integer, name varchar(32) );
CREATE TABLE
load=# create table t3 ( id integer, name varchar(32) );
CREATE TABLE
load=#
load=# insert into t1 values ( 1, 'abc' );
INSERT 3828966 1
load=# insert into t2 values ( 1, 'def' );
INSERT 3828967 1
load=# insert into t3 values ( 1, 'ghi' );
INSERT 3828968 1

load=# select id, max(name) as name from ( select id, name from t1 union select id, name from t2 union select id, name from t3 ) as a group by id;
id | name
----+------
1 | ghi
(1 row)

load=# select id, case when t1.name>t2.name and t1.name>t3.name then t1.name else case when t2.name>t3.name then t2.name else t3.name end end as name from t1 join t2 using ( id ) join t3 using ( id );
id | name
----+------
1 | ghi
(1 row)