Use shared user database table on a multi site

I have made a Drupal 7 multi-site with 3 sites. Each site is running of the same Drupal install and has a separate database.
The database names are myhost_site1, myhost_site2 and myhost_site 3. (My hosting provider puts in the myhost_ automatically).

I need the 3 sites to share the users and user roles. I have read various tutorials and these are the steps I have followed:
1. Created a new database called myhost_users.
2. Copied the following tables from myhost_site1 TO myhost_users:
users, user_roles, authmap and sessions
3. Given the database users for each of 3 main databases access to myhost_users.
4. Open the settings.php for each site and insert the following code (replacing the existing $databse definition):

$databases['default']['default'] = array(
  'driver' => 'mysql',      
  'database' => 'NAME OF DATABASE',
  'username' => 'DATABASE USER',
  'password' => 'DATABASE PASSWORD',
  'host' => 'localhost',
  'prefix' => array(
    'default'   => 'NAME OF DATABASE',
    'users'     => 'myhost_users',
    'sessions'  => 'myhost_users',
    'role'      => 'myhost_users',
    'authmap'   => 'myhost_users',
  ),
  'collation' => 'utf8_general_ci',
); 

5. When I go to my site, I get the following error message:

Uncaught exception thrown in session handler. PDOException: SQLSTATE[42S02]: Base table or view not found: 1146 Table

6. For the code posted in (4) I have tried using an underscore suffix for the database names (e.g, myhost_users_), but I still get an error message.

What am I doing wrong. Do I need any settings in "sites/sites.php"?

I would be open to using a module to do this. Bakery is no good as it doesn't share user roles. Account Sync does, but it doesn't seem to work. CAS requires phpCAS which I do not have access to.