Default username and password for Oracle database

You can simply login as :

sqlplus / as sysdba

Then give password to the respective users :

ALTER USER <username> IDENTIFIED BY <password>;

You can do that for all the three users.

I hope you are not on 12c. Else, you need to mention which CONTAINER are you working with. For example, if you are working on pluggable database, let's say PDBORCL, you need to do following steps :

1. connect / as sysdba;

2. alter session to set the container of which the respective users are a part of.

   alter session set container=PDBORCL;

3. Then follow the same steps to change the passwords for the users.