My answer is based on getting a 403 error although I had all of the Apache settings mentioned in the other answers correct.
It was a fresh Centos 7 server and it turned out that the issue was not the Apache settings but the fact that the PhpMyAdmin did not serve at all. The solution was to install php and add the php directive to apache.conf:
Don't forget to restart Apache server to take effect - systemctl restart httpd.service
I hope this helps. I first thought my issue was Apache directives, so I post my solution here.