An informal categorization of three auction games by Reiner Knizia. Though they were published by different publishers and their releases span 7 years, they are often grouped together, as they represent 3 popular auction games by Knizia.
Dream Factory ("Traumfabrik") has also sometimes been included, making it a tetralogy. Or High Society, making a pentalogy.
See also: Knizia tile-laying trilogy