The California Highway Patrol said three vehicles were involved in the crash just after 1 a.m. on the westbound 91 at Pioneer Boulevard.
A red Jeep rear-ended a gas tanker, then slammed into the center median and landed upside down in the carpool lane, investigators said. Then a black BMW hit the Jeep, according to investigators.
Everyone inside the Jeep was killed.
The Los Angeles County coroner's office identified the victims as Lavell Mann, 33; Legend Mann, a 9-month-old boy; and Logan Mann, a 3-year-old girl.
A GoFundMe page has been set up to help the family with funeral expenses.
A child inside the BMW was hurt with moderate injuries and was expected to be OK.