I initially tested Dungeon Siege III in PlayOnLinux back in 2015 and posted a guide. I was able to finish the game and really enjoyed it!
Now I'm replaying Dungeon Siege III in Steam with Proton and even co-op with my wife and two controllers.
So far Proton is handling the game perfectly! It's even funner playing co-op and the adventure is just as interesting and action-packed as before.
My GeForce GTX 1060 is able to handle the game without any issues and the environments are beautiful!
If you see Dungeon Siege III on sale, definitely pick it up!