The odds might not be great, but it cannot hurt to ask at Fairphone owners in the USA (FP2 FP3 FP4) if someone’s living nearby and might be willing to let you test his or her FP3 bottom module in your FP3.
Other than that, checking the USB-C port for dust/tiny hairs, and trying another cable (if easily available) – would be quite a co-incidence, but you never know.