What did you buy? A new FP3, a new battery, a new cable, a new USB-C port?
Even if it looks clean, it might be worth cleaning it with a toothbrush and isopropyl alcohol. I had problems like the ones you mentioned (intermittent charging, extremely slow charging, and eventually, no charging at all) and was able to fix them by cleaning the port and the contacts.
Another idea is to contact a fairphoneangel (check the angelsmap to see if there’s one in your area) to see if they can help you or try replacing a module, just in case that fixes the issue.
Good luck 