Teetering at the Summit in France
Achieving three-star rating from Guide Michelin no longer guarantees that a chef can run a profitable restaurant in France; some three-star restaurants have had to close, and many others are in financial straits; chefs say that difference between a great two-star meal and a three-star meal is minimal and that Michelin has tended to accord coveted third star to most luxurious places; such restaurants typically require two employees for every diner, and even dinner bills of $500 a person will sometimes fail to cover the costs; photos (M)