create table `a_users` (
    `id` int unsigned NOT NULL AUTO_INCREMENT,
    `user` varchar(32) NOT NULL,
    `pass` varchar(32) NOT NULL,
    PRIMARY KEY (`id`)
);
create table `b_users` (
    `id` int unsigned NOT NULL AUTO_INCREMENT,
    `user` varchar(32) NOT NULL,
    `pass` varchar(32) NOT NULL,
    PRIMARY KEY (`id`)
);
create table `c_users` (
    `id` int unsigned NOT NULL AUTO_INCREMENT,
    `user` varchar(32) NOT NULL,
    `pass` varchar(32) NOT NULL,
    PRIMARY KEY (`id`)
);


insert into a_users values (0,'nathan','password');
insert into b_users values (0,'nathan','password1');
insert into c_users values (0,'nathan','password');



mysql> select * from a_users where user='nathan' and pass='password';
+----+--------+----------+
| id | user   | pass     |
+----+--------+----------+
|  1 | nathan | password |
+----+--------+----------+
1 row in set (0.06 sec)

mysql> select * from a_users where user='nathan' and pass='password' UNION
    -> select * from b_users where user='nathan' and pass='password' UNION
    -> select * from c_users where user='nathan' and pass='password';
+----+--------+----------+
| id | user   | pass     |
+----+--------+----------+
|  1 | nathan | password |
+----+--------+----------+
1 row in set (0.00 sec)

mysql> select * from a_users where user='nathan' and pass='password1' UNION
    -> select * from b_users where user='nathan' and pass='password1' UNION
    -> select * from c_users where user='nathan' and pass='password1';
+----+--------+-----------+
| id | user   | pass      |
+----+--------+-----------+
|  1 | nathan | password1 |
+----+--------+-----------+
1 row in set (0.00 sec)