Are you using a server host (Host Havoc, Vilayer, etc) or are you hosting it on your pc/localhost?
If you are using a server host they should provide you with the database username and password. Whereas if you are hosting it from your PC/localhost you will need to set up a database and use its username and password.
Server host instructions:
Go to your control panel and find a button labeled "Database info", "Show database info", etc.
when clicked it should show something along the lines of:
-- PHPMyAdmin Link --
-- Database Information --
Host: 158.**.***.** *
Database Name: ***
The script has executed successfully. You may close this window.
Go into your exDB3 config and replace
IP = 127.0.0.1
Port = 3306
Username = arma3
Database = altislife
IP = database IP
Port = 3306 (leave as is or change to database port)
Username = database username
Password = database password
Database = database name
Then save your exDB3 config and attempt to run your server.
To set up a local host database I recomend following the GitHub instructions