Your finished block with 1/2" seams will be 5" (1/2" lost on each side). I never sew the X in the blocks. Since all 3 layers are caught in the seams there is no problem with them staying together. (Where would they go?) The purpose of the X is if you're using a smaller inside filling, which doesn't get caught in the seams and therefore can bunch up. Someone said that she had a problem with the bulk at the corners, and with one of the layers shifting while she was sewing, but she was using two layers of flannel and one of a more slippery, thicker fabric. I don't think you will have any problems with 3 layers of flannel. When you're done with the sewing you clip all the seam allowances, including into the corners. Be sure to show your quilt when done, looking forward to seeing it.