This volume constitutes the thoroughly refereed post-proceedings of the Third International Workshop on Algorithm Engineering and Experimentation, ALENEX 2001, held in Washington DC, USA in January 2001. The 15 revised full papers presented together with the abstracts of three invited presentations have gone through two rounds of reviewing and revision and were selected from 31 submissions. Among the topics addressed are heuristics for approximation, network optimization, TSP, randomization, sorting, information retrieval, graph computations, tree clustering, scheduling, network algorithms,...