While two of them died on the spot, one died on his way to the hospital.
Organizers of the annual event, called Larry's Fun Fest, said about 5,000 people attended Sunday's show.
At least two of the three victims were from Aurora, Illinois, WDJT-TV writes.
UPDATE (WKOW) - The hunt is on for the shooter who killed three people at a Wisconsin racetrack Sunday night. The men were standing near a food vendor when a gunman approached and shot them at what appeared to have been point-blank range, Beth told reporters.
The suspect was believed to be a black man in his mid-20's wearing either a black hoodie or white t-shirt, with hair shaved on the sides of his head and grown out at the top.
The three were shot to death Sunday evening, August 13, 2017 at Great Lakes Dragaway in Union Grove.
Beth said he could not give a motive for the shooting. "After he assassinated the three subjects he was seen running through the crowd and fleeing in a vehicle", Beth said.
Malecki said authorities are trying to figure out what precipitated the shootings and who was involved.
"I'm thinking this was a mass exodus though I can't say for sure".
There was armed private security on site; the track was not screening people for weapons, Beth said, but the incident happened in a parking area where anyone could have driven in. It took several hours to get everyone out.