Codeigniter Join with Multiple Conditions

I'm using Codeigniter Active Records class and I want to join my users table with my clients table, so that I can display the users' "real" names and not just their ID.

Here is what the clients table looks like (example) and the columns a_1, a_2, and a_3 correspond to my users table, specifically, the user id:

  clients
|--------|---------|----------|-----------|----------|
|  id    |   name  |   a_1    |    a_2    |   a_3    |
|--------|---------|----------|-----------|----------|
|   1    |   Paul  |    2     |     4     |    1     |

And my users table looks like this:

   users
|--------|---------|----------|
|  id    |  first  |   last   |  
|--------|---------|----------|
|   1    |   Paul  |  Blake   |
|--------|---------|----------|
|   2    |   Dan   |   Doe    |
|--------|---------|----------|
|   3    |  Steve  |  Smith   |
|--------|---------|----------|
|   4    |  Harry  |  Jones   |
|--------|---------|----------|

So, essentially, if I were select from the clients table and JOIN it, it would look like this:

  clients
|--------|---------|----------|-----------|----------|
|  id    |   name  |   a_1    |    a_2    |   a_3    |
|--------|---------|----------|-----------|----------|
|   1    |   Paul  | Dane Doe |Harry Jones|Paul Blake|

Thus far, I have tried (which hasn't worked, it just displays the same name for all):

<?
 $this->db
  ->select('name, a_1, a_2, a_3')
  ->from('clients')
  ->join('users', 'users.id=a_1 OR users.id=a_2 OR users.id=a_3');

I got the solution interms of MYSQL Query. I am finding it hard to convert the query into CI Active Record. But try this:

$sql = "SELECT `name`, CONCAT(`u1`.`first`,' ', `u1`.`last`) as a_1, 
                       CONCAT(`u2`.`first`,' ', `u2`.`last`) as a_2, 
                       CONCAT(`u3`.`first`,' ', `u3`.`last`) as a_3 
        FROM `clients` 
        LEFT JOIN `users` as `u1` ON (`u1`.`id`=`a_1`)
        LEFT JOIN `users` as `u2` ON (`u2`.`id`=`a_2`)
        LEFT JOIN `users` as `u3` ON (`u3`.`id`=`a_3`)";

$result = $this->db->query($sql);