U.S. auto sales rose 3% in March over a year earlier, but new warning signs emerged that car companies are stretching to keep demand humming after record results last year. March’s selling pace came in at a disappointing adjusted annual rate of 16.57 million light vehicles, well below analyst expectations and the 17.5 million clip the industry reported in February. Detroit’s three auto makers each reported sales gains, but their results missed expectations, sending shares lower. Auto makers overall sold 1.6 million light vehicles in March. Discount spending is on the rise and new evidence emerged in March that sales to fleet customers such as rental agencies in some key cases boosted new-car tallies. Car loans stretching 84 months or longer and the share of vehicles leased both increased, according to researcher J.D. Power. […]