The boy rang the doorbell and returned to the Prius they were riding in, and the group drove off.
Three boys, Daniel Hawkins, Jacob Ivascu and Drake Ruiz, were killed, the authorities said.
The driver, Sergio Campusano, then 18, and two other boys, Joshua Hawkins, then 13, and Joshua Ivascu, then 14, were injured.
He also testified that he had drunk 12 bottles of beer on the night of the crash, the newspaper reported.
“The murder of these young men was a horrendous and senseless tragedy for our community,” Mike Hestrin, the Riverside County district attorney, said in an emailed statement.