New CEOs are set to have a big impact at these companies.CEOs make a difference. And for different reasons, investors in these three companies will hope that their new leaders will make a tangible difference in improving performance.William Brown took over as CEO of 3M (MMM -1.65%) on May 1 and has significantly influenced investors' thoughts about the company. Kelly Ortberg will take over as CEO of Boeing (BA -1.09%) on Aug. 1, and Johnson Controls (JCI -1.41%) recently announced it's searching for a new C ...