The Grant County Sheriff’s Office said it received a report of a shooting shortly before 8.30pm on Saturday at the site near the small city of George, 149 miles (239km) east of Seattle. Officers pursued the suspect, who was then taken into custody.
The names and conditions of the victims were not immediately available.
Organisers of the Beyond Wonderland electronic music festival said on social media that Sunday’s events at the Gorge Amphitheatre had been cancelled.
The shooting occurred on the campsite adjacent to the concert stages, the sheriff’s office said.
Concert organisers described the location as an “overflow camping area”.
KREM-TV reported the gunman as being among the three people injured. This was according to Kyle Foreman, public information officer with the sheriff’s office.