Hi there Floriane! Seam 3 is the first seam you make in the waistband- you fold the fabric (with right-side of fabric to the inside) inside so the short edges meet, sew the seam, then turn the waistband right-side out- you have a circle of fabric now. You now fold this circle of fabric lengthways on itself with right-side on the outside, so you have a nice neat fold on top and the raw edges together at the bottom. I hope this helps, it is hard to explain, easier to see in pictures- did you find the tutorial I put up? If you missed it, here it is