Steam Deck natively recognizes the PS3 controller. Simply plug it in via USB, wait for Steam OS to recognize it, then unplug USB cable and it will work over Bluetooth without needing
Amazon.com: EasySMX 2.4G Wireless Controller for PS3, PC Gamepads with Vibration Fire Button Range up to 10m Support PC (Windows XP/7/8/8.1/10), Steam, PS3, Android, Vista, TV Box Portable Gaming Joystick Handle :