FrozenByte has announced Trine 3: Artifacts of Power, and is due out for PC later this year.
Following the tried-and-tested franchise template, Trine 3 will focus on solving physics-based puzzles by switching between the three playable characters: thief, king and wizard; each featuring unique abilities. The official description pretty much confirms this by stating "The Three Heroes embark on a new adventure through an enchanting fairytale world full of wonders, puzzles, dangerous foes and strange magic."
Unlike its predecessors however, in which you played a 2D game with 3D graphics, Trine 3 seems to feature full 3D segments, allowing you occasionally to move between the foreground and background.
Adding a third installment and making Trine a trillogy makes sense, since the game has sold up to 7 million copies, to date.
Trine 3: Artifacts of power is slated for release in 2015. FrozenByte hasn't announced the platforms, but all previous titles landed on Xbox, PlayStation 4, and PC.