A Venn diagram is a diagram that depicts all logical relations between a finite collection of sets. Union Circle is the name of the set of all possible intersections between two given circles. A Venn diagram 3 Union Circle is a diagram that depicts all logical relations between three finite collections of sets. The diagram shows the overlap of three circles in the Venn diagram. The circles represent three sets of data. A set is a group of things that are similar in some way. The overlapping area between the three sets signifies how many things are common to all three sets of data.