It seems like it is impossible to play DS3 with a steam controller and this mod:
sfx.thelazy.net/games/preset/5431/
It's a ReShade preset. Not sure if the incompatibility lies in something you guys, Steam, or both would have to address but I wanted to point it out.
For what it's worth, I'm playing with an old controller and ReShade until this gets sorted out because it makes the already great-looking game absolutely fantastic
ETA: The ReShade plugin also seems to interfere with other Steam functionality. I can't use either the F12 or SteamButton+RT .shortcuts to take screenshots. If I launch the game via Big Picture mode, a lot of the time (maybe all of the time? Haven't tested extensively) the ReShade settings don't take.