MASPEGHI 2004 is the third edition of the MASPEGHI workshop. This year the organizers of both the ECOOP 2002 Inheritance Workshop and MASPEGHI 2003 came together to enlarge the scope of the workshop and to address new challenges. We succeeded in gathering a diverse group of researchers and practitioners interested in mechanisms for managing specialization and generalization of programming language components. The workshop contained a series of presentations with discussions as well as group work, and the interplay between the more than 22 highly skilled and inspiring people from many different communities gave rise to fruitful discussions and the potential for continued collaboration.