Many websites (example) indicate that the Fairphone 3 has an IP54 protection (which means that it has good protection against dust, and some protection against water), although I haven’t found that information in the official page of Fairphone (even though it was commented in this forum and it seems that at the time there was this specification).
However, I have been using the phone for several months now and in my experience, the phone adequately resists normal situations such as quick use unden the rain, or a sweaty or even something wet hand when picking up the device after taking a shower, for example. I think the phone doesn’t have a problem with this kind of things in a daily use. In addition, I have the impression that the bumper included is an extra protection in these situations as well 
Take a look to this official support article from Fairphone: https://support.fairphone.com/hc/en-us/articles/115001535486-Is-Fairphone-dust-or-water-resistant-