PostgreSQL really doesn't put much effort into supporting moves from newer major versions back to older ones. One big reason for that is that newer versions usually support features which you just can't install on older versions; although any given database might not be using any of the new features, the project philosophy is that they would rather not support it than to support it in a buggy or incomplete fashion.
Your best bet might be to generate a "plain" format dump rather than a "custom" format dump. This is just a text file of SQL commands. If you aren't using any new features, it might just apply cleanly. If not, you can look into the errors and decide what to do for a back-port.
You can dump the whole PostgreSQL cluster with pg_dumpall. That's all the databases and all the globals for a single cluster. From the command line on the server, I'd do something like this. (Mine's listening on port 5433, not on the default port.) You may or may not need the --clean option.
$ pg_dumpall -U postgres -h localhost -p 5433 --clean --file=dump.sql
This includes the globals--information about users and groups, tablespaces, and so on.
If I were going to backup a single database and move it to a scratch server, I'd dump the database with pg_dump, and dump the globals with either
pg_dumpall --globals-only, orpg_dumpall --roles-only (if you only need roles)
like this.
$ pg_dump -U postgres -h localhost -p 5433 --clean --file=sandbox.sql sandbox
$ pg_dumpall -U postgres -h localhost -p 5433 --clean --globals-only --file=globals.sql
Outputs are just text files.
After you move these files to a different server, load the globals first, then the database dump.
$ psql -U postgres -h localhost -p 5433 < globals.sql
$ psql -U postgres -h localhost -p 5433 < sandbox.sql
I thought pg_dumpall would at least backup foreign keys, but even that
seems to be an 'option'. According to:
http://www.postgresql.org/docs/9.1/static/app-pg-dumpall.html even
with pg_dumpall I need to use a -o option to backup foreign keys
No, that reference says "Use this option if your application references the OID columns in some way (e.g., in a foreign key constraint). Otherwise, this option should not be used." (Emphasis added.) I think it's unlikely that your application references the OID columns. You don't need to use this option to "backup foreign keys". (Read the dump file in your editor or file viewer.)
Best Answer
I would run the backup jobs on A and B at specified times using the local native pg_dump, then push them to C for storage.
Restoring from the backup file can be done with: