Step 6: Sew in third sleeve
Sew sleeve in place, stitching right inside the edge of the split seam. (you'll likely have extra bits of cut sleeve protruding beyond, which is fine.)
Turn the outfit right-side out to admire/check your work.
Now you can sew your third arm into the sleeve, anchoring it at the shoulder only for extra-lifelike flop. Alternatively, go put the suit on your 3-armed baby.