How tha handle this.
My client wants one shared catalog, and at least three different shops using parts of that shared catalog.
See it as one shop based on the shared catalog, and three brand specific shops, based on the " master" catalog.
1) Using services. Enable the service on the master catalog, and providing updates to the brand catalogs.
2) Shared tables. All different installs, but sharing the catalog tables. The adjusting the views, or using domain-access for the brand catalogs