Sorry for a late response.
During the Perfect Server installation, I have checked phpmyadmin and realized that it worked as is. But right after I installed ISPconfig 3, I started getting 403 errors when trying to connect to phpmyadmin page.
I have checked everywhere, even read through this message. Then looking at the Apache configurations, I have realized that there was a lower-upper case problem for phpmyadmin.
The actual name of the folder is /usr/share/phpMyAdmin, while the /etc/httpd/conf/sites_available/ispconfig.conf file reflects /usr/share/phpmyadmin (all lower case. I have corrected the name and restarted Apache. Ta daaa.
There are some parts of the installation that are neglected or forgotten. I have noticed a couple of them during installation. I'll list them here, so that the original document might be updated accordingly.