There isn't a password for PT3 but PostgreSQL may need one. Did you install PostgreSQL? Click Start --> Programs --> PostgreSQL --> Start Services to start the PostgreSQL service. If you don't see the "Start Services" link or the PostgreSQL folder then you don't have PostgreSQL installed and you will need to download and install PokerTracker 3 Beta 26 Full Install (which contains PostgreSQL) from our download page.
If you have PostgreSQL installed, the default values for the database are:
Server: localhost
Port: 5432
User: postgres
Password: dbpass
If this password does not work for you, you can disable password authentication by following the directions of this tutorial.