GASOLINE PRICES fell sharply on the New York Mercantile Exchange Wednesday in response to a higher-than-expected buildup in U.S. stocks. Pump prices for drivers are now expected to fall by a national average of 3 or 4 cents - and by up to 10 cents a gallon in the most competitive areas - within a few weeks . . . THE CHURCH of Scientology expanded its campaign against the anti-depression drug Prozac Wednesday with a full-page add in USA Today. The church has been battling the drug's maker through its own media and in other ways for several years, because the Scientologists oppose psychiatry in general and prefer their own methods of altering behavior.