Repetition amounts to emphasis. That the “mouth” of each creature is separately listed means that we need to pay special attention to their mouth. John draws attention specifically to what comes out of their mouths. As we know from experience, what comes out is words (Matthew 15:18–20).
13 And I saw, coming out of the mouth of the dragon and out of the mouth of the beast and out of the mouth of the false prophet, three unclean spirits like frogs.