This volume contains the proceedings of FroCoS2000, the 3rd International WorkshoponFrontiersofCombiningSystems,heldMarch22-24,2000,inNancy, France. Like its predecessors organized in Munich (1996) and in Amsterdam (1998), FroCoS2000 is intended to o?er a common forum for research activities related to the combination and the integration of systems in the areas of logic, automateddeduction,constraintsolving,declarativeprogramming,andarti?cial intelligence. There were 31 submissions of overall high quality, authored by researchers from countries including Australia, Brasil, Belgium, Chili, France, Germany, - pan, Ireland, Italy, Portugal, Spain, Switzerland, The Netherlands, the United Kingdom, and the United States of America. All submissions were thoroughly evaluatedonthebasisofatleastthreerefereereports,andanelectronicprogram committeemeetingwasheldthroughtheInternet.Theprogramcommitteesel- ted 14 research contributions. The topics covered by the selected papers include: combinationoflogics;combinationofconstraintsolvingtechniques,combination of decision procedures; modular properties for theorem proving; combination of deduction systems and computer algebra; integration of decision procedures and other solving processes into constraint programming and deduction systems. We welcomed ?ve invited lectures by Alexander Bockmayr on "Combining Logic and Optimization in Cutting Plane Theory", Gilles Dowek on "Axioms vs. Rewrite Rules: From Completeness to Cut Elimination", Klaus Schulz on "Why Combined Decision Problems Are Often Intractable", Tomas Uribe on "Combinations of Theorem Proving and Model Checking", and Richard Zippel on "Program Composition Techniques for Numerical PDE Codes". Full papers of these lectures, except the last one, are also included in this volume.