Postgres data integration Consider a scenario where I have 3 standalone running Postgres engines
that are installed on three separate machines.

The data on this machine is inserted, updated or removed each month by employees. Changes made to the database must be sent and integrated into a master engine by the means of a usb drive (all the work is done offline).

One way is to always copy all the data from the 3 machines each month and then copy them to the master machine. Is there a more practical solution or technology to that on Postgres?
Try the dblink. You find the description in http://www.postgresql.org/docs/current/static/contrib-dblink-function.html and http://www.postgresonline.com/journal/archives/44-Using-DbLink-to-access-other-PostgreSQL-Databases-and-Servers.html. Also a question about dblink was asked on https://stackoverflow.com/questions/3862648/how-to-use-install-dblink-in-postgresql.