I wanted to store rdf triples in three different databases.
How do I configure the "Oracle SPARQL Service Endpoint using Joseki", So that I should be able to query with sparql queries to get triples from these databases?
You need three different SPARQL endpoints deployed. One endpoint for each Oracle database.
To query these endpoints, you can use programming APIs through Jena/Jena Adapter. You can
also use SPARQL service call to query multiple endpoints together in a single SPARQL query.