By default, Minix 3 does not have a root password. That is an obvious security problem, especially if the system is being used for any serious purpose. So, you should set a root password, as soon as possible, unless you're just experimenting on an isolated machine.
To change the root password, log in as root, and run the command
passwd. You'll be asked to type a new password. You won't be able to see it, as you type, but don't worry. Type your new password, and hit enter. You'll be asked to retype it, in order to check for typing errors – once you've done so, hit enter again.
# passwd Changing local password for root. New password: Retype new password: #