What is the password to switch user to root. I just put the new ad card into rpi3b and in trying to su root I am asked for a password and I enter raspberry and get authentication error.
root user on Raspbian doesn't have a valid password at all. This is an intentional security measure which prevents someone from logging in as
root to your board. Raspbian comes with a default
pi user who has a password
raspberry; once you change that password your RPi gets some basic security. Having a default
root password would require you to change passwords for both users in order to secure your RPi.
You can set
root password by running
sudo passwd. This is only recommended if you want to play with login scripts of your
pi user or
sudo settings - having a
root password will enable you to login and rescue your system if you accidentally lock yourself out of your
pi account or break
sudo (the latter specifically can only be fixed if you login as
root). Otherwise I wouldn't do it.