This volume constitutes the refereed proceedings of the Third International Conference on Foundations of Software Science and Computation Structures, FOSSACS 2000, held as part of ETAPS 2000 in Berlin, Germany in March/April 2000. The 25 revised full papers presented were carefully reviewed and selected from a total of 68 submissions. Among the topics covered are rewriting, pi-calculus, lambda-calculus, formal languages, proof systems, type theory, mobile processes, automata theory, predicate logic, etc.