The airplane was substantially damaged at 0907 Central time during an impact with terrain after takeoff. The Commercial pilot and two passengers were fatally injured; a third passenger was seriously injured. Visual conditions prevailed. The surviving passenger later stated the airplane seemed sluggish and wasnt picking up speed during the takeoff roll. After liftoff, the airplanes stall warning sounded and the pilot slightly leveled the airplane, silencing the warning. The passenger stated that the right wingtip went up high and the nose went down simultaneously when the stall warning horn sounded for the second time.