This book constitutes the thoroughly refereed post-proceedings of the Third International Workshop on Formal Approaches to Agent-Based Systems, FAABS 2004, held in Greenbelt, MD, USA in April 2004.The 18 revised full papers and 4 revised poster papers presented have gone through two rounds of reviewing and improvement. Among the topics addressed are agent management systems, norms in agent societies, multi-agent systems reliability, agent architectures, multi-agent systems specification, formal methods, potential energy in multi-agent systems, timed automata, model checking, OBDDs, multi-agent systems verification, agent communication protocols, autonomic agents, and Petri nets.