Grant Gustin teases Back to the Future moment in The Flash season 3 We already know things are going to be very different for Barry Allen in the third season of The Flash, and just like in the Back to the Future series (a staple for the executive producers of the show), not only will Barry find himself in a strange world but he might start to forget about the one before. “Barry’s the only one that knows the timeline is different,” Grant Gustin said in the latest episode of DC All Access.