does the event which offered at the beginning of section 3 is a coincidence, according to Sgt.Major's quote ?

The realization by Mrs. White that she still has two wishes left makes her wish that Herbert would return to life not surprising, but the fact that they hear a knock on the door (when there is wind outside) could be a coincidence, or it could be the fulfillment of Mrs. White's wish. If it is the wind, then it is, indeed a coincidence.