JOIN the tables first, then build your WHERE clause. By the way, is that POST form data you are using? If so you should escape the untrusted user data first. An example using PHP:

$location_id = mysql_real_escape_string($post['location_id']);

Now it is safe to use in your query statement:

$sql = "SELECT 
  f.id, 
  f.firstname, 
  f.lastname, 
  f.tel, 
  f.mobile, 
  f.fax, 
  f.email, 
  f.focus_id_fk, 
  focus.focus_id, 
  focus.focus, 
  focus.afr_focus 
FROM franchisee as f 
INNER JOIN franchisee_region_int AS i ON(f.id = i.franchisee_id_fk) 
INNER JOIN focus ON(f.focus_id_fk = focus.focus_id) 
WHERE 
  i.location_region_id_fk = '{$location_id}' 
ORDER BY f.firstname" 
;