Not sure of the rest, but ServerAlias doesn't have http:// in it and you don't need to alias the same ServerName. Also, I am pretty sure you need to have all your options on one line.

 <virtualhost *:80="">ServerAdmin webmaster@example.com    ???
    ServerName mhb.501clients3.com
    ServerAlias www.mhb.501clients3.com
    DocumentRoot /srv/www/501clients3.com/clients3_new/mhb
    ErrorLog /srv/www/501clients3.com/logs/error.log
    CustomLog /srv/www/501clients3.com/logs/access.log combined
    <directory srv="" www="" 501clients3.com="" clients3_new="" mhb=""># This relaxes Apache security settings.
        AllowOverride all
        Options FollowSymLinks -MultiViews</directory></virtualhost> 

Not seeing your .htaccess it is hard to tell if you are turning on the rewrite engine etc…

It looks like the message board added the 'http://', please disregard.

This is the htaccess file:

BEGIN WordPress

RewriteEngine On

RewriteBase /

RewriteRule ^index.php$ - [L]

RewriteCond %{REQUEST_FILENAME} !-f

RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule . /index.php [L]

END WordPress

I've got the correct owner and permissions set on the htaccess file too.