Apple increased its share of both the U.S. smartphone platform and overall U.S. cellphone markets in the three months ending in May, according to the latest data from comScore. Apple was the third-largest cell phone manufacturer during the period, capturing 15 percent of the market—up 1.5 points from February—and trailing only Samsung with 25.7 percent and LG, which accounted for 19.1 percent of the market. Motorola and HTC rounded out…