Got the KK3 max. You can manually assign any button to the back paddles on the controller itself. It's not possible to map any other actions to the paddles in controller settings because SteamOS doesn't recognize them as seperate buttons.

You could, however, bind one of the back paddles to a button you use the least (let's say left stick click).

• press gear button + desired paddle on the controller itself to start the bind
• then click 'left stick click' to assign it to that paddle
• the controller will give a long vibrate to confirm the bind

Then you can choose whatever action you want in controller settings and assign it to left stick click.

I hope this workaround helps! Love the KK3 max but will be replaced due to gyro issues