... The first two working group reports (on Extrasolar Planets and the Herschel--ALMA Synergies) were released in 2005 and 2006, and this third report covers the area of Fundamental Cosmology. The Working Group's mandate was to concentrate on fundamental issues in cosmology, as exemplified by the following questions: . What are the essential questions in fundamental cosmology? ... What are the appropriate methods with which these key questions can be answered? ...