"We believe this incident was domestic violence-related," Brookfield Police Chief Daniel Tushaus said at a news conference.
All three of those shot to death were female, Tushaus said. Their ages were not immediately provided. The police chief declined to say whether Haughton's wife was among the victims.
Haughton was found dead of an apparent self-inflicted gunshot wound at the Azana Salon & Spa, Tushaus said.
The shooting marked the second outburst of gun violence at a U.S. beauty spa in a matter of days. On Thursday, a gunman opened fire at a salon in suburban Orlando, Florida, killing three women and wounding another before fleeing and committing suicide at a nearby residence.
According to Wisconsin court records, a temporary restraining order related to domestic abuse was issued against Haughton on October 8 in Milwaukee County. On October 18, an injunction was granted prohibiting Haughton from possessing a firearm, and he was ordered to hand over all firearms to the sheriff.
Haughton also was cited for misdemeanor disorderly conduct, described as domestic abuse, stemming from an incident on January 8, 2011. The charge was dismissed on June 15, 2011.
Sunday's shooting erupted at about 11 a.m. local time, according to eyewitness accounts.
Joe Brent, a 26-year-old Milwaukee man, said he was leaving a fast-food restaurant near the salon when he heard a gunshot. Within about 2 minutes, a policeman entered the restaurant and told everyone to leave, he said.
He added that as he was getting into his car he saw a woman who appeared to be in her 20s with a bloodied paper towel on her neck being rushed from the salon and onto a stretcher. "She was screaming. She was hurt real badly," Brent said.
Four victims with gunshot wounds were rushed to Milwaukee's Froedtert hospital. Their conditions were not immediately known.
President Barack Obama was informed of the shooting at 1:30 p.m. EDT, according to a statement from the White House. The statement said that the president and first lady Michelle Obama's "thoughts and prayers" were with the victims.
Wisconsin Governor Scott Walker issued a statement calling the shooting another example of the "senseless acts of violence" that occur across the country.
"Our state will stand with the victims and their families," he said.
The last few months have been marked by a string of mass shootings in the United States. On September 27, a disgruntled former employee killed six people and took his own life in a shooting rampage at a Minneapolis sign company from which he had been fired.
In August, two people were killed and nine wounded in a work-related shooting near the Empire State Building in New York City. The Manhattan incident followed two separate mass shootings in a Colorado movie theater and a Sikh temple in Wisconsin.
Nationally, there were 458 workplace homicides in 2011 and 518 in 2010, according to U.S. Bureau of Labor Statistics data.
