This has nothing to do with Seam at all.
There are three configurations because these are three distinct parts of Java EE (and in one case Seam).
You have to have a persistence unit descriptor (persistence.xml or hibernate.cfg.xml) to use JPA or Hibernate.
You don't need the JCA data source configuration, though it is recommended. As an alternative, you can put your database connection information inside of the persistence unit descriptor, thus reducing one configuration file.
Seam has to know how to hookup with the persistence unit to use a Seam-managed persistence context. To eliminate this configuration, use @PersistenceContext on a SFSB.
So, its possible to get it down to one configuration, the persistence unit descriptor.