You cannot do that natively with Postgres. However, one solution would be
to make the archive_command into a separate file, that the main
postgresql.conf includes. Then you can keep copying the postgresql.conf but
keep the archive_command separate and unique. The syntax is
include = 'myserver.conf'
And then the myserver.conf would contain things specific to each server,
e.g.
archive_command = 'test ! -f /archives/INST3/%f && cp %p /archives/INST3/%f'
cluster_name = 'inst3'
Cheers,
Greg