UPDATE 1-'Thunder' still No. 1 at North American box office

LOS ANGELES, Aug 31 (Reuters) - Action film parody "Tropic Thunder" clung to the top spot at the North American box office for a third straight week as the summer moviegoing season sputtered to a lackluster close, Hollywood studios reported on Sunday.
Paramount Pictures' farcical combat movie within a comedy, starring Ben Stiller, Robert Downey Jr. and Jack Black, sold an estimated $11.5 million in U.S. and Canadian tickets Friday through Sunday to bring its three-week tally to $83.8 million.
While the final weekend heading into the U.S. Labor Day holiday is typically one of the slowest of the summer, the box office was especially lethargic despite five new films competing for attention in domestic theaters. None of those even managed to even crack the $10 million mark.
"It was an underwhelming end to a phenomenal summer," said Paul Dergarabedian, head of box office tracking service Media By Numbers.
Business also was likely dampened by the approach of Hurricane Gustav along the U.S. Gulf Coast, where many families were too busy boarding up their homes and fleeing to higher ground to go to the movies.
"Tropic Thunder," about a group of self-absorbed actors who get caught up in a real-life battle with narco-terrorists while filming a war movie in Southeast Asia, was the only movie to post ticket sales in the double-digit millions.
Its biggest competition came from a real action flick, the sci-fi thriller "Babylon A.D." from 20th Century Fox starring Vin Diesel, which grossed an estimated $9.7 million in its first weekend to land at No. 2.