Are you sure the USB C port has clean contacts, did you try and disassemble the actual module to better clean it. FP3 Bottom & Top module breakdown
Do you have a fairphoneangels near you that may a bottom module you can try?
Try charging without any SIM or SD card, but don’t try powering on without them.
Have you tried a safe mode start?
If you can save you personal data you could try a factory reset, starting with section
Fairphone does not boot anymore.
If your Fairphone does not boot into Android anymore, you will have to perform the reset using the system recovery. Choose your phone below to see the relevant instructions: