Are you sure that you have created three database instances rather than three schemas? Oracle schemas are roughly comperable to SQL Server databases.
When you invoke the export utility, you specify the database instance to connect to (i.e. scott/tiger@instance). If you don't specify an instance, the default instance will be chosen.
The import utility will not create a new database instance. It will import the data into an existing database.
Justin
Distributed Database Consulting, Inc.
http://www.ddbcinc.com/askDDBC