The black soul stone scene in Leah's dreaming cinematic contains the three prime evils. During Adria's betrayal, as scene in the boss portraits here (http://www.d3sanc.com/wp-content/uploads/2011/11/Diablo-3-Boss-Portraits.png), she inserts the soulstone into herself, thus becoming the three headed feminine Diablo figure (not Leah!).
He wasn't talking about prime evils being in cinematic. He was talking about black soul stone in cinematic.