This is what i think will happen in season 3 of the Flash.

Kid Flash, Captain Cold, Vibe and Dark Archer will be heroes while Mirror Master, Doctor Alchemy, Killer Frost and Reverse Flash will be villains. I also think we will see many characters different from what we saw of them first(just like on Earth-2) I also think that Barry will lose his powers at the beginning of the first season but get them back during the next episode. But I still have no idea how they're going to pull of the 4 parted crossover.