The building was quickly locked down and dozens of people were seen streaming out with their hands raised. Two nearby schools also were locked down but no one on the campuses was hurt.
"One male gunman shot one female, one male and then apparently took his own life," Police Chief Dennis Smith told reporters.
The conditions of the two people he wounded were not immediately known. There was no immediate word on what prompted the gunfire.
Multiple media reports said the gunman was an Edison employee and his two victims were believed to be company managers. Police said they could not confirm that.
The shootings took place about 1:30 p.m.
The utility's office is in a complex of buildings that also includes a business called California Lighting Sales.
Cindy Gutierrez, the controller for that company, said employees there didn't hear any shots fired and didn't realize anything was amiss until building management announced over the intercom that everyone should stay indoors.
"At that point we knew something was wrong, then 5 to 10 minutes later that's when we hear the police," she said, adding that she and her 20 colleagues have been locked in their office ever since.
"I'm fine, kind of nervous," she said.
Both of the schools ended the lockdowns about two hours after the shooting and began releasing students.
The complex has two buildings and Gutierrez's company's office is on the western end. The Edison building is on the eastern end.
She called the campus "pretty safe," with a guard and amenities that include a cafe and gym.
Irwindale is a small industrial city of about 1,400 residents in the San Gabriel Valley, 22 miles east of downtown Los Angeles.
It is home to the popular Irwindale Speedway auto racetrack. It is also home to the annual Southern California Renaissance Pleasure Faire as well as sprawling rock and gravel quarries.
Southern California Edison is one of its largest companies, employing 2,100 people.