From the Wikipedia page 
Triangle Region Denmark (Danish: Trekantområdet) is a cooperation consisting of seven Danish municipalities on the Danish peninsula of Jutland and the island of Funen: Billund, Fredericia, Haderslev, Kolding, Middelfart, Vejen and Vejle.
The Triangle Region began as the general term for the industrial and communications hubs of Kolding, Vejle and Fredericia in the early 1960s. The three cities (thus ‘triangle’) originally worked together to coordinate and collaborate locally, but have since expanded into a cooperation between seven municipalities, which together have 421,480 inhabitants (as of 1 January 2018) and cover an area of 4,266.0 km2.
An area of political and economic stability, the Triangle Region is often regarded as Denmark’s third major region after Aarhus and Copenhagen. It has a reputation for having lower levels of pollution, crime and traffic problems than the big cities.