You can add new functionality to your mouse by going to the Accessibility setup. Here's how:

1. Make sure your mouse is connected to your iPad.
2. Go to Settings > Accessibility > Touch > Assistive Touch > Devices > MX Master 3 > Customize Additional Buttons.
3. If you want to customize buttons on your mouse, a message will appear and you'll be prompted to enable AssistiveTouch.

    

NOTE: These customizations aren’t meant to help with the workflow but to help people with disabilities to have quicker access to some elements.