Even though they didn't have much to do in Series 2, I still think they made the season better. They're part of Broadchurch, the town won't feel the same without some of the townspeople. I really hope they return, they added depth to the show.
http://deadline.com/2016/04/broadchurch-season-three-final-new-cast-david-tenant-olivia-colman-charlie-higson-georgina-campbell-itv-bbc-america-1201735963/
At the National Television Awards in January, Charlotte Beaumont (Chloe) was with Andrew Buchan and Jodie Whittaker and they commented that they would ALL be back. Just because Nige, Chloe, Olly, Lucy, Becca, or any other previous character/actor was not mentioned does not mean they're not in it. They didn't announce these five for series 2 and they were all in it.
The press release was chiefly to highlight some of the new cast and to confirm Colman and Tennant would be back.
I'm betting we'll also see Hardy's ex-wife and daughter return, though they weren't announced.