I’ve used my Samsung III for many years in a rain case while biking (with komoot as Navi). Now I am stuck with looking for a replacement of that rain case which seems pretty difficult due to the incompatible and unusual shape. A bit skeptical I still hope there will be an acceptable alternative available until August 2020 (another 40 days bike tour). Appreciating any idea …
Have a look at one of those, maybe there is one that suits your needs. I don’t know exactly about their delivery conditions though.
I have the M version that fits well for my FP2, so there is also a L version which may fit for your FP3.
They have them also sealed for water resistance and if you wish with touch sensitive top foil as well. It’s a matter of price after all.
After using it more than 1,5 years now I am still very satisfied with it and cannot see signs of wear.
A recommendation would be however to make the padding of the bottom layer a bit thicker. While riding the phone may get a bit too much shaking depending on which ground your are riding on and if your bike is spring borne.
But as the pad layer can be easily folded out of the pocked there is a good option, which I have done, to slide another pad into it before folding the original back into place. An easy fix for that minor weakness if one even sees it as such.