According to a recent article on Marvel.com, Henry Simmons is returning to the role of Mack in Agents of S.H.I.E.L.D. next season, despite suggestions in the final episodes of Season 2 that he would be leaving. This casting news adds to recent news that Luke Mitchell's character, Lincoln, has been promoted to a series regular for Season 3.
We can also safely assume that the characters of Lance Hunter and Bobbi Morse, as portrayed by Nick Blood and Adrianne Palicki, will be returning, given how the spinoff allegedly featuring them got canceled. The only other cast members that seemed like they wouldn't be returning were Ming-Na-Wen's Melinda May and Elizabeth Henstridge's Jemma Simmons. Of course, they didn't get proper goodbyes, so they're bound to return in some form next season. The character of Melinda May is probably having a post-[The Avengers: Age Of Ultron](tag:293035) Hawkeye-esque break.
Clark Gregg, Chloe Bennet and Iain De Caestecker as Director Phil Coulson, Skye/Daisy Johnson and Leo Fitz are all confirmed to return.
It looks like most of the cast is returning, with Brett Dalton promising big things from Grant Ward. Of course, Ruth Negga (Raina), Jamie Harris (Gordon) and Dichen Lachman (Jiaying) aren't returning, while Kyle MacLachlan appearing again hasn't been ruled out.
J. August Richards as Mike Peterson/Deathlok and Patton Oswalt as Agent Koenig haven't been confirmed, but given their tendency to reappear, they could possible make an appearance.