Hi maur094,
Thanks for reaching Microsoft Surface Community. We're sorry to hear you are having issues with the brightness on your Surface Laptop 3. Definitely this is not the experience that we want you to have.
Does this happen even if the power supply is plugged in?
Please try these steps to troubleshoot the issue:
Step 1 : Run the Surface Diagnostic Toolkit
Step 2 : Open Settings > System
Under Display, uncheck "Change brightness automatically when lighting changes"
Step 3 : Open Control Panel
Hardware and Sound > Power Options
> Change plan settings > Change advanced power settings
Expand Display > Turn off display
On battery > change it to Never
Plugged in > change it to Never
Click Apply > OK
Please let us know if this helps. This will help other members who may have the same issue as you. Otherwise, don't hesitate to update us by posting or replying.
Kind Regards,
Marrion Y.