I really think the third yellow top girl is Sara Harvey, not Bethany. Maybe I just don’t want to believe they are going to bring yet another person back from the grave. I do think Sara would make more sense.
We know that Bethany passed enough as Alison to get whacked that night. Assuming she was murdered because someone thought she was Alison. But as far as I can remember, we don’t know a whole lot about Bethany’s personality. However, we know that Sara basically was Alison. The queen bee, the group of followers, etc. Sara’s story mirrors Alison’s.
It was said that the last time Sara was seen alive was the day after Labor day. This is a day after Alison went missing. By this time, Alison had already went into hiding. I believe A took Sara because she was just like Alison and has been “playing” with her (that sounds dirty but I mean it doll wise) ever since. A couldn’t have the real thing so they took a replacement. Like many children believe, you can never have too many dolls. Maybe A is a collector of Alison dolls.
We have seen a picture of Sara, but on the wiki page it says that the picture was a stock image. So it’s obvious they’d have someone else playing her role if they decided to bring her into the story. CeCe was there “that night”. She’s also been said to play the role of Alison, dressing in her clothes and pretending to be her. If CeCe has disappeared and can’t be found, maybe A has kept Sara around to play the part of CeCe. The promo makes it look like Alison is there, recreating that night also. If so, maybe she’s being forced to play herself, and then Sara and Mona as Bethany and CeCe.