As the FP3 is constructed differently, there is no such thing as a slim case.
And from the experience with the FP2 all those 3D-printed cases were not really slim ones.
Take a look at these threads:
And as the FP3 is so far in the wild only in small quantities, there hardly is enough information to design a user case. Especially since there are no design files released up to now.
And it’s on Fairphone to decide if they ever wil release those files. The files for the FP2 for example were removed from the download page: