The area of autonomous agents and multi-agent systems (MAS) has grown into a promising technology offering sensible alternatives for the design of distributed, intelligent systems. Several efforts have been made by researchers and practitioners, both in academia and industry, and by several standardisation consortia in order to provide new languages, tools, methods, and frameworks so as to establish the necessary standards for a wide use of MAS technology. The papers of this volume focus on the development of programming languages and tools that can effectively support MAS programming and the implementation of key notions in MAS in a unified framework to bridge the gap from analysis and design to effective implementation that way. This book constitutes the thoroughly refereed postproceedings of the Third International Workshop on Programming Multi-Agent Systems, ProMAS 2005, held in Utrecht, The Netherlands in July 2005 as an associated event of AAMAS 2005, the main international conference on autonomous agents and multi-agent systems. The 14 revised full papers presented together with 2 invited articles are organized in topical sections on multi-agent techniques and issues, multi-agent programming, and multi-agent platforms and organisation.