If you are hand stitching the binding to the back, most quilters cut their double folded binding 2.25". The only time I ever cut binding 3" was when I was using some fat polyester batting. Check to make sure you are sewing an accurate 1/4" seam when attaching your binding to the quilt. Most quilters (Not ALL) who cut their binding 2.5" width is because they are stitching it totally on by machine.