Eyewitness News has learned the incident was being investigated as a possible murder-suicide. Authorities were not searching for any suspects.
Investigators said there was some sort of argument before the gunfire began on the 9900 block of Marshall Street at about 7:30 p.m.
A neighbor told Eyewitness News that she spoke to the wife of the suspected gunman. She said the man shot his daughter and her friend.
"He was just getting tired of it, going constantly and constantly just arguing, so he just got tired and he just shot his daughter on the side once, and he shot his daughter's girlfriend twice, and then he started going down the street and he shot himself," said neighbor Dianna Salinas.
L.A. County Sheriff's homicide detectives were assisting the El Monte Police Department in investigating the case. Police say there was a witness.
"What we know at this point is that one of the victims is the daughter of the male suspect. The eyewitness could be the daughter of one of the female victims," said Lt. Eddie Fernandez with the El Monte Police Department.
All three victims were pronounced dead at the scene. The suspected gunman was in his 50s, and the two female victims were in their 30s.