I've never used pgadmin3, but I have restored PG databases from the command line many times using the psql client. Was the source created by running pg_dump?
Usually the "postgres" user has no password, either at the OS level or the database level.
Suppose the dump file is called mydatabase.dump. I would follow these steps:
If that does not work, try using "psql template1 < /path/to/mydatabase.dump".
[enter your password]
psql < /path/to/mydatabase.dump
I do use Microsoft Access as a GUI frontend from time to time after installing the ODBC driver for PostgreSQL in Windows. That can be useful for importing large datasets in a spreadsheet or CSV/TSV formats. Mostly I just run queries from the command line using psql.