I usually just quess, if too much, I save the extras for scrappy quilts. For the size quilts I normally make I cut 9 strips - 2 1/2 inches wide and as long as the width of fabric is - selvage to selvage. If I am follwing a pattern, I cut as many strips as it says. Three inches would be way too big, 2-1/2" wide should be good enough - fold in half and press.