Hi there.. you want to create process instances using classes? or process definitions using classes?
For jBPM 3, you can read the book: https://www.packtpub.com/jboss-business-process-management-jbpm-developer-guide/book
that explains how to use the APIs to interact with your processes.
jBPM 3 is quite old these days and most of the questions that we are answering here are for jBPM5.
Cheers