The Chevy suburban and Honda sedan crashed around 9:30 p.m. on Sunday on Baseline Road at Cherry Avenue.
The suburban, carrying the three brothers, left the road and crashed into a power pole. Two of the brothers died at the scene, another died in the hospital.
Their names have not been released, but the coroner identified the men as a 35-year-old male from Rancho Cucamonga, a 39-year-old male from Bloomington, and a 37-year-old male from Fontana.
Noah Ferrer, 11, who was a passenger inside the Honda, died in surgery.
The Fontana Police Department is investigating the crash.