This book constitutes the refereed proceedings of the Third International Workshop on Experimental and Efficient Algorithms, WEA 2004, held in Angra dos Reis, Brazil in May 2004. The 40 revised full papers presented together with abstracts of two invited talks were carefully reviewed and selected from numerous submissions. The book is devoted to the areas of design, analysis, and experimental evaluation of algorithms. Among the topics covered are scheduling, heuristics, combinatorial optimization, evoluitonary optimization, graph computations, labeling, robot navigation, shortest path algorithms, flow problems, searching, randomization and derandomization, string matching, graph coloring, networking, error detecting codes, timetabling, sorting, energy minimization, etc.