How to deal with diffrent doc types in solr?

I’d like to index three different types of documents in solr: DocumentA, DocumentB, DocumentC. Some of the fields/attributes in the document types are equal, some are different. The documents have relations to each other. What’s the best pratice to index them with solr?

Use a single core for alle 3 doc types or should I use 3 cores? I’d like to be able to search over the three doc types at once and have them ranked dependelty from each other