Thank you for config information. Three tables have relation (by foreign key) ?
You might want to have one nested <entity> tag in <document> rather than 3 <entity> one in <document>. By using nested <entity> tag, you may able to merge tables *before* importing them to Solr. All works done by SQL. You have already seen this wiki? If not, examples in this page will help you. http://wiki.apache.org/solr/DataImportHandler#Configuration_in_data-config.xml
