The Trine series is uniformly excellent in 3D, and an absolute must-play. However, Trine 3 features a depth of field effect that takes away somewhat from the excellent backdrops. This fix removes it.
Installation:
1) Download this ZIP file
2) Extract it to the game's directory, where the exe is. For Steam, this is \Steam\steamapps\common\Trine 3
3) Run the game and in the launcher, select 32bit. It won't remember this choice, so you'll have to do it every time you play.
Full credit to DarkStarSword, who told me how to fix this.