Search a value in several tables PHP + Mysql

I'm doing some maintenance on a system which has several tables for each access level.

I need to verify a field called IID in every table so I can see the person access level.

Who made the code used three separated searches:

$var->exeQuery("SELECT iid FROM leve11 WHERE iid=$iid");
$var->exeQuery("SELECT iid FROM leve13 WHERE iid=$iid");
$var->exeQuery("SELECT iid FROM leve12 WHERE iid=$iid");

I know that I can make it using a JOIN, but I'm doing something wrong because nothing is returned.

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     SELECT iid FROM level1 WHERE iid=$iid
     UNION ALL 
     SELECT iid FROM level2 WHERE iid=$iid
     UNION ALL 
     SELECT iid FROM level3 WHERE iid=$iid

or if the info is different and you want to join instead of union....

    SELECT level1.iid, level2.iid, level3.iid FROM level1
    LEFT JOIN level2 
    ON level1.iid = level2.iid
    LEFT JOIN level3
    on level2.idd = level3.iid
    WHERE iid=$iid

select
    t.iid, l1.iid as Level1, l2.iid as Level2, l3.iid as Level3
from
    master_table t
left join
    level1 l1 on t.iid = l1.iid
left join
    level2 l2 on t.iid = l2.iid
left join
    level3 l3 on t.iid = l3.iid
where
    t.iid = $iid

The user has any level that does not return Null as the result.

How about something like this:

select iid from level1
inner join level2 on level1.iid = level2.iid
inner join level3 on level3.iid = level2.iid
where level1.iid = $iid