For McDonald’s, the third quarter saw steady international growth, but the same worrying trend at home — with the burger giant struggling to keep meals cheap enough for cash-strapped, low-income Americans. To wit: Global comparable sales rose 3.6%, the company said Wednesday, marking only a slight slowdown from 3.8% last quarter. Meanwhile U.S. same-store sales rose 2.4%, slipping from 2.5% last quarter and again led by “positive check growth,” not increased traffic. That’s industry shorthand for higher s ...