Well maybe sb. else can help regarding the mysql users created with ispconfig 3 as I do not know the process.
Besides, if i log in phpmyadmin with root / rootpass does work (and i see mysitedb), if i test the user "dbadmin" in command line, works fine against mysql and i can access "mysitedb", the problem is when i try to log with that user over phpmyadmin to access its database.
So how is your mysql server configured in phpmyadmin? When configuring it to use "localhost" it ignores the port and uses the local socket instead (as the mysql client does on the commandline). If entering the IP-Adress (127.0.0.1 or the external host IP) it does not.
If then the mysql user you are referring to is added with no remote access allowed the authentication will fail.
So what you should check, is the "Host" entry in the User-table of the mysql database for the referred users:
SELECT Host, User FROM `user`;
localhost means no remote connection, whereas e.g. % means connections from anywhere.
After that you should check the configuration of your mysql server in phpmyadmin.