I think I pretty much figured out how Merle will first appear in season 3. In the first episode close to the end, Andrea and Michonne are hiding from the woodbury soldiers,then she sees a gun pointing at her head from behind, she turns around slowly and sees it's merle, who says How's about a hug for your old pal merle?

Then the credits come, leaving a cliffhanger.

I figured this out because in the trailer for a second a gun's pointing at Andrea and she turns around slowly and looks shocked.