On June 8, 2017, about 1630 central daylight time, a Head Balloons AX 77B balloon, N30721, encountered a wind gust while preparing for takeoff that threw the balloon occupants to the floor of the basket. The pilot and one passenger received minor injuries and one passenger received serious injuries. The balloon received minor damage to the basket. The balloon was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not on a flight plan. The flight was originating from Chatsworth, Illinois when the accident occurred. The pilot reported that before the flight, he sent a small helium balloon up which indicated favorable conditions for flight. He proceeded to layout and cold inflate the balloon. He then performed a preflight briefing with the passengers and hot air inflated the balloon. He stated that the winds were still light, and after completion of the preflight checklist the passengers boarded the basket. As he started to add more heat to the balloon, he heard people begin yelling and he stopped burning to see what was going on. He saw the balloon beside him was airborne and swinging around beside its chase truck. The accident pilot's balloon which was still cold suddenly became airborne and swung very rapidly around the side of his chase van and back in the opposite direction it was laid out in. He looked down and realized spectators had moved down beside his van and were now directly below the balloon. He immediately started burning to add heat to the balloon so it would rise so that it would not come down on the spectators. The balloon now swung out away from the spectators and hit the ground and drug for a short distance before becoming airborne again. After becoming airborne again, the balloon began swinging on its tie off line and was hitting the ground again. When it hit the ground, the tie off became uncoupled. When the basket hit the ground again, the pilot was throw-out of the basket and was hanging by his heels and from a rope with the balloon airborne. When the basket hit the ground the next time, the pilot was thrown from the basket and the balloon having lost his weight flew on with the two passengers in the basket. It landed approximately two blocks east of the launch field where the two passengers jumped from the basket. After losing their weight the balloon flew on for about one fourth of a mile and came to rest in a soy bean field. Weather reports for the Pontiac Municipal Airport, Pontiac, Illinois, about 20 miles northwest of the accident site recorded the wind speed and direction as 5 knots from 220 degrees about the time of the accident. Videos taken by a ground observer showed the wind condition prior to and during the event. In one video, there are several inflated balloons seen and no indication of strong winds. Another video showed several balloons being blown about by high wind. A predominately blue balloon is seen colliding with the accident balloon with its basket scraping the accident balloon's envelope. The blue balloon then clears the accident balloon and lifts off. The accident balloon's burner can be seen burning in an apparent attempt to climb. The balloon climbs a few feet with winds causing the balloon to reverse course several times. The balloon then strikes the ground several times. After one impact an occupant can bee seen hanging from the side of the basket. During the next impact the hanging occupant can be seen to fall to the ground and the balloon then flies off. During the entire video the wind is pushing the balloon in different directions with the envelope shape distorting from the wind. The balloon flies off past a building and then is seen descending behind the building. The witness reported that the wind was calm just prior to the gusts that could be seen in the accident video.