* Third-quarter U.S. growth slips from prior reading * HP shares tumble after weak profit outlook * Indexes: Dow flat, S&P up 0.1, Nasdaq up 0.2 pct *Trading volume is likely to be thin this week as global uncertainties and the U.S. Thanksgiving holiday keep many investors on the sidelines. Campbell Soup Co reported first-quarter earnings that beat expectations while sales were slightly below consensus, sending the stock down 4.5 percent to $32.11.