This book constitutes the refereed proceedings of the Third International Workshop on Algorithms and Computation, WALCOM 2009, held in Kolkata, India, in February 2009. The 30 revised full papers presented together with 4 invited papers were carefully reviewed and selected from 102 submissions. The papers feature original research in the areas of design and analysis of algorithms, computational geometry, graph drawing and graph algorithms. The papers are organized in topical sections on computational geometry, graph algorithms, complexity, graph drawing, approximation algorithms, and randomized algorithms.