You can probably skip step 2. Shutdown stops the service as well. But yeah, the built in PS3 pairing system doesn’t really seem to work.
It appears as if the service doesn’t get added to the boot process automatically unless the PS3 pairing script finishes successfully (which I’ve never gotten it to do). So once you install the PS3 drivers, then plug in the controller and run..
sudo /opt/retropie/supplementary/ps3controller/sixpair
and get the two MAC addresses to match, confirming the top MAC address (the controller) matches the bottom MAC address (your Bluetooth Dongle), you can then go ahead and add the sixad daemon to the startup sequence.
sudo sixad --boot-yes
Unplug the USB cable, give the Pi a reboot, and it should sync up after it boots up when you hit the PS button.