PS3 Controller SCP Toolkit Driver - how to install properly

Do not forget to unplug the controller once, after you have successfully installed the driver like shown here. I replicated this step to make sure.

You can unplug it even while running Grim Dawn. After playing a short sound it should start working immediately. I recommend restarting the computer after any driver installation, but it does not seem necessary for the controller software to connect.
Once the controller gets connected you will notice the red bulbs numbering 1 to 4 on your controller will stop blinking as well.

You do never need to install the dualshock 4 driver (unless you have one), or the bluetooth driver if you plan to play with the cord.

As I mentioned, I haven't looked into how to make bluetooth work. As far as I know you would need to install the driver for bluetooth, as well as some physical bluetooth dongle on your computer. If you have any advice on this and want to share, I can give credit and link it to my guide.

Even if it is greyed out as you are not using a steam controller, if you click on the button "configure steam controller", it will push you to desktop and bring up the steam controller settings like shown in the next picture below.

You can change some controls with the Grim Dawn keybindings, but some buttons will only be recognized through the steam controller settings.

If you want to make personal changes, you can swap here between game and UI (user interface) controls to make changes. I do not think it is necessary. Just make sure to click the button I highlighted to import the preset as seen in the picture below.

CLICK ON THE SCREENSHOT TO ENHANCE!

After you have loaded the mentioned steam controller preset for Grim Dawn above, you want to be able to individually assign your skills for each character quickly within the game.

To do this, press the select button on your PS3 controller to bring up your character screen.
Use your forefinger/pointer on the R1 button to navigate from the character screen to the skills tab. Then you just need to press your triangle button to bring up the menu.
It looks like a wheel menu to assign your skills for each button.

You can navigate this wheel menu with your arrow cross on the left side of your controller, or the game auxiliary function key L1 on the left (joystick rotation).

To bring up a list of all skills, navigate to the button you want to assign the skill to, then press the X button and scroll through the menu with your cross up and down keys to select your skill choice. Then confirm your selection by pressing X once more.

In the list of skills is also a selection marked with "X" >clear this quickslot button<, to remove any aciton from said key. You can always close the wheel menu or any other menu with your O button.

Final mention, you may feel confused when trying to assign buttons to your square and X button:
The square button on your controller is named X in the game, while the X button on your controller is named A. Either get used to it or try to change it through the steam controller configuration.