Hi AngelYoung2,
Thanks for reaching out to Microsoft Community and posting your inquiry. We're sorry to hear that the backlights of your Surface Laptop 3 Keyboard isn't turning on.
The backlight key of the Surface Laptop 3 should be the F1 key. You can keep pressing the key without using the FN key. That should enable the keyboard backlight. With each press it moves through 4 levels of brightness, including off.
If this answers your question, we highly encourage you to inform our Community by clicking on Yes or marking my post as an answer. This will help other members who may have the same issue as you. Otherwise, don't hesitate to update us so we can further assist you.
Kind regards,
John Neil_J