Hello!
I just want to give you a feedback on my experience in case it could help someone here.
For several weeks, I had difficulties to plug my USB-C wire into the phone: no charge or slow charge at 1st, 2nd or even 3rd plug, before the charge begins. Another (minor) issue I had was that the torchlight delivered a variable light and not a stable one (as I don’t use it very often, I don’t know when it began but I suspect that it was at the same time).
After trying external cleaning of the USB-C port (with toothpick, alcohol on a cotton bud and an air spray), I had no real result, so I decided to open the phone and clean it from the inside. And that was the good option!
I followed this guide to open and extract the bottom module: Fairphone 3 Bottom Module Replacement - iFixit Repair Guide. Then I carefully cleaned the module and the rest of what I saw dirty inside the phone, using alcohol on cotton buds and air spray, before reinserting the bottom module and closing firmly the phone with the screws.
A thin metal strip broke when I removed the connecting cable of the bottom module and I didn’t succeed in putting it back in place… but apparently that was not absolutely necessary as… everything now works perfectly fine again!
Let’s go for 6 years more with my FP3? 
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. And if it becomes very painful, I’ll try a bottom module replacement (if I can find one)!