The suspected shooter at the Grand Sierra Resort was among those transported to a hospital, district health officer for the Northern Nevada Public Health Department Dr Chad Kingsley said.
Two victims were in critical condition, while the other had been released from the hospital, Sparks Police Chief Chris Crawforth said during a news conference.
The suspect had no known connection to the victims, police said, adding that the motive is unknown.
The shooter walked up to the casino-hotel's valet parking area, pulled out a handgun and pointed it at a group of people, police said. His gun initially malfunctioned, but he quickly was able to get it to shoot.
The suspect fled through the parking lot where he encountered an armed casino security guard. Crawforth said the gunman opened fire on the guard, who returned fire.
The suspect also shot at someone driving by in the parking lot, striking and killing the driver.
Officers arrived less than three minutes after the first shots rang out and fired at the suspect, police said in a statement.
"Tragedies like this are horrific for any community to endure," Reno Police Department Chief Kathryn Nance said.
The resort is one of Reno's most prominent venues, hosting concerts, sporting events and a campaign rally by President Donald Trump before the 2024 election.
Near the California border and just northeast of Lake Tahoe, the town is a popular summer tourist destination.
Hotel guests heard several shots coming from the resort's valet area about 7.30am.
Michael Sisco, 60, said he was in his room and about to head to the valet for his car when he heard popping sounds.
He looked out his window and saw people screaming and running from the valet area.
He said he saw a man holding his stomach and staggering before falling motionless next to a car.
Sisco moved away from the window as the gunshots continued.
"A half hour after the incident I started shaking because I realised I could've been there," he said.
"That's exactly where I was going."
Hours after the shooting, the entrance inside the casino leading to the valet area was roped off to guests and privacy screens were placed in front of the glass doors to block the view outside.
In addition to those who were killed and the people taken to the hospital, three more were treated for minor injuries at the resort, according to Devon Reese, chair of the Board of Health in northern Nevada.
One person was grazed by a bullet on their pinky finger, one was treated for severe anxiety and one was injured while running, he said.
10°C