The men were discovered outside a southern Louisville apartment complex Sunday afternoon; two of them were inside a Ford Explorer that had a shattered window, and the third was on the ground outside the vehicle, The (Louisville) Courier-Journal reported.
The victims were not identified.
Police said they had yet to determine a motive for the slayings and that no arrests had been made.
"Specifically what occurred to lead up to this incident, we're still trying to determine," Louisville Metro Police spokeswoman Alicia Smiley said.
Detectives declined to say whether any weapons were found at the scene.
It is the first triple homicide in Louisville this year.
"I think anytime we have a tragedy like this, it highlights the need for the work to keep moving," Anthony Smith, director Louisville's safe neighborhoods working group that works to curb violence in the city, said. "This isn't an overnight kind of change, but we've got to keep at it."