As seen on this promo-pic for Season 3, Branson is standing along with all the other main characters. This might mean that he is now billed as a series regular.

Also, look at how Matthew and Isobel is no more on the left, with all the servants; now we have Sybil there.

And about this 'beloved character' that will die in Season 3. My guess is Patmore, Molesly, Hughes, Violet or Isobel. Patmore, Hughes, Violet and Isobel is all loved by fans, and they are all old. Molesly has his lung issues, and he is also a loved character.