I should explain that although NodeXL can be used to show a Venn diagram (and in fact is used for that in another CodePlex project), it's not very easy because you have to do the Venn-related size and position calculations yourself. I'm sure there
are other tools out there that are more appropriate for the task.
But if I haven't scared you away yet, please see the Venn Diagram Demo topic on NodeXL's Issue Tracker tab at
http://nodexl.codeplex.com/workitem/19375. It includes a NodeXL workbook that shows a Venn diagram.
The workbook uses a layout algorithm called Polar Absolute, which in turn uses polar coordinates on the Vertices worksheet to specify the vertex locations. The problem is that you have to compute those polar coordinates yourself, along with the vertex
sizes, based on the data represented by the three vertices. That's certainly doable, but it's not something that NodeXL will do for you.
There is a formula for determining the vertex Size to specify to give a vertex a specified radius in screen units. I can tell you that formula if you decide to go ahead with this.