Since early May, insider buying activity has surged at three major companies. Together, top executives and influential insiders have invested more than $170 million of their own money, sending a strong signal of confidence in the future of these stocks. Here’s a closer look at the three companies attracting serious insider attention. Get UnitedHealth Group alerts:Board Chairman Signs +$55 Million Check for BLDR Shares Builders FirstSource TodayBLDRBuilders FirstSource$126.90 +0.90 (+0.71%) 52-Week Range$10 ...