Here's a diagram that may help. If you print it out so it's 3" square, the middle wedge should give you the size template needed. I didn't know how big you wanted the circle so two sizes are included. You can also draw it up yourself if you make a 3" square as I included the measurements for the lines. Don't forget to add seam allowances when cutting out the fabric.
[ATTACH=CONFIG]611409[/ATTACH]