In the third legal action so far in connection with the Dec. 23 crash of a light plane into a Concord, Calif., shopping mall, the parents of Mario Molina Jr., 3, filed suit in Contra Costa County Superior Court against the mall owners, the pilot and the manufacturers of the Beechcraft. The child was in the arms of Santa Claus when the plane plunged through the roof of the shopping center, severely burning him. Five people died and 75 were injured. The Molina boy remained in fair to serious condition at Children's Hospital in Oakland. Attorney Melvin Belli filed the action on behalf of the family.