This book constitutes the thoroughly refereed post-conference proceedings of the Third International Symposium on Combinatorial Optimization, ISCO 2014, held in Lisbon, Portugal, in March 2014. The 37 revised full papers presented together with 64 short papers were carefully reviewed and selected from 97 submissions. They present original research on all aspects of combinatorial optimization, such as algorithms and complexity; mathematical programming; operations research; stochastic optimization; graphs and combinatorics.