Police found two of those killed at a science building on the Casper College campus, and the third at another location about 2 miles away. The suspect wasn't believed to be a student, Police Chief Chris Walsh said, but few other details about the identity and those of the victims were released, except that two were male and one was female.
Walsh said it appears there's a relationship between the suspect and those killed.
The attack occurred just before 9 a.m. Friday at the two-year community college. Class was in session and the campus was locked down after the attack and the science building was evacuated, but the lockdown had ended by early afternoon.
Casper, population 56,000, is about 250 miles northwest of Denver.
Police provided some details in a news conference streamed live by the Casper Star-Tribune (http://trib.com/ ).
School spokesman Rich Fujita said he had no additional information, including how the attack happened, what type of weapon may have been used, or who was killed.
Political science instructor Chris Henrichsen said he was showing the film "Frost/Nixon" to his Wyoming and U.S. government class when he stepped into the hall to get something for a student and was told a homicide had occurred on campus.
He went back to his classroom, where students were getting messages about the campus lockdown on their phones.
"We locked the door and waited for further instruction," Henrichsen said.
The class finished watching the remaining 10 minutes of the movie and remained locked down 20 minutes after that, he said.
The students then were sent home, but some who parked near a different campus building where the attack occurred had to leave their cars there, Henrichsen said.
Henrichsen said he saw nothing unusual or suspicious on campus before the attack.
Casper College is a two-year community college in Wyoming's second-largest city. Casper, population 56,000, is about 250 miles northwest of Denver.
Wyoming residents refer to Casper as the "Oil City" because it's a hub for the state's small oil industry. The city is flanked to the south by Casper Mountain, scene of a massive wildfire in September that burned more than 24 square miles of forest and meadows and destroyed 37 homes and cabins.
Associated Press writer Ben Neary contributed to this report.