This book constitutes the refereed proceedings of the Third Internatio nal Workshop on Frontiers of Combining Systems, FroCoS 2000, held in N ancy, France, in March 2000. The 14 revised full papers presented toge ther with four invited papers were carefully reviewed and selected fro m a total of 31 submissions. Among the topics covered are constraint p rocessing; interval narrowing; rewriting systems; proof planning; sequ ent calculus; type systems; model checking; theorem proving; declarati ve programming; logic programming; and equational theories.