A three Venn diagram is a visual tool for illustrating the similarities and differences between three groups of objects or numbers. This Venn diagram is a graphic organizer used to show the similarities and differences between groups of objects or numbers. A Venn diagram, in other words, is a visual tool for comparing and contrasting two sets. A three Venn diagram, such as the one shown below, is familiar to most people. The red area on the left of this three Venn Diagram is filled with facts or characteristics only true of A. The right-hand blue area is filled with attributes or characteristics unique to B.