The Nevada Department of Public Safety said the wounded victims were in surgery and one of them is a doctor.
Police initially said the gunman shot four people before killing himself, but they later said he wounded two people and killed another before committing suicide.
"We have two people confirmed deceased in the building, two others injured," said Reno Police Deputy Police Chief Tom Robison.
Department of Public Safety spokeswoman Gail Powell said the wounded doctor is a woman but she had no more information about the identity of the victims or the gunman, the type of weapon or a motive for the attack. She said the shooting happened at the Center for Advanced Medicine and Neurology of Nevada.
Renown Regional Medical Center was put on lockdown amid the chaos of the shooting as law enforcement rushed to the scene and victims were treated by doctors on the hospital campus. More than three dozen police cars, including SWAT team vehicles, surrounded the sprawling medical complex and closed off a three-block area near downtown Reno.
State Sen. Debbie Smith said she was at Renown for an appointment and was trying to leave a nearby building around 2:45 p.m. when the shooting occurred.
"I encountered some SWAT team guys, they said nobody was leaving," she said. "A police officer was at the door and said I couldn't go out there."
Renown Regional Medical Center posted a notice on its website less than two hours after the shooting saying that operations had returned to normal at the main hospital but police were still investigating the shooting in an office building on the northeast corner of the campus.
The Center for Advanced Medicine building is a modern four-story structure connected by a second-floor walkway to the hospital and a parking structure.
Gov. Brian Sandoval said on Twitter that he and his staff were monitoring the situation and his thoughts and prayers were with those affected by the tragedy.
Renown Regional Medical Center is the largest hospital in northern Nevada. It has played an important role in responding to several recent tragedies in northern Nevada, including a crash at a Reno air race in 2011 that killed 11 people and a rampage at a Carson City restaurant the same year that killed three uniformed Nevada National Guard members.