He's just not in episode two. They're doing things in a different order from the book it seems, since I recall that Clegane wasn't captured until after Dondarrion was re-introduced at their secret hideout. And Thoros wasn't brought back until the hideout too, whereas in the show they're introducing him early. But everything that's happened so far hasn't been at the secret hideout yet, I don't think.

But yes, there is an actor for Beric Dondarrion in season three. There are photos of him out there. He'll be having the same fight scene as in the book, since that's part of the pictures.