This text constitutes the refereed proceedings of the Third International Conference on the Unified Modelling Language, 2000, held in York, UK in October 2000. The 36 revised full papers, presented together with two invited papers and three panel outlines, were carefully reviewed and selected from 102 abstracts and 82 papers submitted. The book offers topical sections on use cases, enterprise applications, applications, roles, OCL tools, meta-modelling, behavioural modelling, methodology, actions and constraints, patterns, architecture, and state charts.