Joshua Acosta, 23, claimed he was trying to save his teenage friend, Kaitlynn Goodwill-Yost, from an abusive family situation.
He was convicted Nov. 8 of three counts of first-degree murder, along with the special circumstance allegation of multiple murders, for shooting the three victims on Sept. 23, 2016.
Frank Felix, Acosta's friend and co-defendant, is still awaiting trial. He also faces life without parole if convicted.
Police said the men met the stepmother and her daughter through "furry" and "bronies" gatherings.
City News Service contributed to this report.