This volume contains the proceedings of FroCoS'2000, the 3rd International Workshop on Frontiers of Combining Systems, held March 22--24, 2000, in Nancy, France. Like its predecessors organized in Munich (1996) and in Amsterdam (1998), FroCoS'2000 is intended to offer a common forum for research activities related to the combination and the integration of systems in the areas of logic, automated deduction, constraint solving, declarative programming, and artificial intelligence. The topics covered by the selected papers include: combination of logics; combination of constraint solving techniques, 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.