* They: If you're talking about multiple faces, "they" is the most common and neutral pronoun. Example: "The faces in the crowd were all looking at the stage. They seemed excited."
* Their: If you're talking about something belonging to the faces, use "their." Example: "The faces in the photo were all smiling, showing their teeth."
* Them: If you're using "faces" as the object of a verb or preposition, use "them." Example: "I saw them all looking at me."
It's important to understand the context and the function of the pronoun in the sentence to choose the correct one.