Authorities said three people have been arrested, CNN reported.
The other two wounded fans were in stable condition, police said.
A fourth person may also have been injured, but left the parking lot next to Sports Authority Field without seeking treatment, said Denver Police Sgt. Steve Warneke.
The incident, about 10 p.m. local time, began as a scuffle, but officials said the cause of the fight wasn't known.
The Chargers defeated the Broncos 27-20.