3XN is a Danish architecture studio which evolved from Nielsen, Nielsen & Nielsen (later 3X Nielsen), founded in Arhus in 1986 by Kim Herforth Nielsen, Lars Frank Nielsen and Hans Peter Svendler Nielsen. Lars Frank and Hans Peter later left the company. Today the practice is lead by Principal Architect Kim Herforth Nielsen in partnership with Bo Boje Larsen and Jan Ammundsen.

3XN was created as a limited company in 1995. The practice had its international breakthrough in the late 90s with the Danish Embassy in Berlin (completed in 1999) and the Muziekgebouw Concert Hall in Amsterdam. The firm's head office is located in Copenhagen, where approximately 80 employees operate in a project-oriented network structure.