Pillsbury Best, the third all-purpose flour in supermarket sales nationally, is hardly flowering in Phoenix, where Brand Marketing checked prices at three stores. Its 10.3% share of the $321 million market around the country dropped to 6.3% locally for the 52 weeks ended Feb. 27, 1994. Its business fell 35.9% compared with the year before, according to Information Resources Inc. The leader, Gold Medal from General Mills, holds a 35.6% share in the United States, followed by private label with 15.4%. However, they respectively rise to 55.2% and 21.5% locally. The three retailers priced Pillsbury Best the highest of the three brands.