Turn the paper back over to the PRINTED side, holding the fabrics in place while you do so. Sew along the line between pieces #1 and #2 (no need to backstitch). Snip threads off.
Take the pin out. Fold the paper back along the seam you have just sewn. Trim the seam allowance to ¼”.
Open piece #2 right side up. Finger press the seam (or you can use the tip of your iron).
Cut a piece of fabric large enough to cover piece #3. Place it right sides together over piece #1, aligning edges.
Turn the paper back over to the PRINTED side, holding the fabrics in place while you do so. Sew along the line between pieces #1 and #3 (no need to backstitch). Snip threads off and trim seam allowance to ¼”.