This book constitutes the refereed proceedings of the Third International Workshop on Principles and Practice of Semantic Web Reasoning, PPSWR 2005, held in Dagstuhl Castle, Germany in September 2005. The 12 revised full papers presented together with 3 invited contributions were carefully reviewed and selected for inclusion in the book. The major aspects of semantic Web research are addressed in the papers, namely semantic Web architectures, language issues, and formal reasoning methods. The advances are investigated in the context of new design principles and challenging applications