when I set mine up, I measured 5.25" for the inner pleat. Then sew down the pleats. Now if you go back and measure that inner pleat, its going to be more like 5", between the stitches. You loose .25" when you sew the pleats.
The outer pleats, because they are pressed will still measure 3" edge to edge, outside the stitch.
If your still not sure grab a 10" scrap of fabric and try a couple of practice pleats.... you'll see how it goes.