Naomi Bettis called emergency services before the shootings, but the dispatcher, citing the state's gun laws, said Noah Harpham had a right to carry the rifle outside and hung up.
Ms Bettis called back when Harpham killed a bicyclist outside her door.
Harpham went on to kill two more people on the streets of Colorado Springs.
Police later confronted him and shot him dead. Officials told the Colorado Spring Gazette that the dispatcher "did not deviate from policy".
The dead included two women relaxing outside an addiction recovery centre and the cyclist, who begged for his life.
"His last words were 'Please God, no,'" said Teresa Willingham, a witness to the first shooting. "He was just at the wrong place at the wrong time."
Ms Bettis said she was not sure if the threat of violence was real or not because it was on Halloween, but she wanted to tell the 911 dispatcher what she saw.
"I don't think she probably thought it was an emergency until I made the second call, and that's when I said, 'That guy I just called you about, he just shot somebody,'" Ms Bettis told the The Washington Post.
The dispatcher's response "blew me away, like she didn't believe me or something," said Ms Bettis.
Carrying a firearm in public is legal in many states in the US.
The gunman had suffered from alcoholism, which his mother, Heather Kopp, outlined in her book entitled "Sober Mercies: How Love Caught Up with a Christian Drunk".
He posted a strange video online two days before the shooting talking about his anger at his father, but did not indicate any violence to come.
A motive for the attack remains unknown.
More from the BBC - Have Colorado's new gun laws failed?
In the wake of the 2012 Aurora theatre massacre, Colorado Governor John Hickenlooper signed laws banning the sale of ammunition magazines with more than 15 rounds and broadening the requirements for background checks before gun purchases.
Twenty months after that controversial law went into effect, the magazine ban seems not to be working as intended.
And in a western state where guns are said to be part of the way of life, the politicians who called for gun control are on the defensive. Watch Franz Strasser's report from Colorado above.
Colorado police did not heed a warning from a woman who reported seeing her neighbour outside with a rifle before he killed three people.