Check the Fairphone site or phone info. Not all phones support external video. It’s a hardware thing. Doesn’t matter if it is USB-C or microUSB, Android 5 or Android 10/11. Of my devices, the Essential PH-1 (USB-C, /e/OS Pie), OnePlus 8T (USB-C, stock OxygenOS Android 11), and LG-G3 (microUSB, multiple ROMs - KitKat thru Oreo) have video out. The rest do not. Kind of a bummer considering my ZTE Axon 7, something of a multimedia device with dual front-firing speakers, Dolby ATMOS, and USB-C, has no video out.
Searching the web I get conflicting info. There’s no technical info from Fairphone showing it supports DisplayPort. There are articles out there saying you can use (or try) MHL or SlimPort adapters for video out. That can be an expensive what-if though.
I personally don’t think the Fairphone 3 supports video out. Maybe other FP users can say otherwise.