A matrix diagram is great for comparing three different variables at the same time. Circles are positioned on the diagram as they correlate to the X and Y axis variables and the variable is represented through the size of the circle.
A matrix diagram accepts one Row Field, one Column Field and one Measure data columns for input.
Remember to select an Expression for Measure. You can do this by clicking on the field first and then choosing from the drop down. If you select non-numeric data, the Expression will be Count.