Because I discussed a lot about for FP2, I want to let You know my experience about the FP3 also:
Just received my feuerwear Mitch15 and it fits for FP3 WITH protection case.
without pumper and protection case I would take the Mitch14.
Just to let You know and they are good. I’m NOT affiliated with the company.
Strange, I just sent Mitch 15 back because it is definitely too small for FP3 with bumper.
For my FP3 Mitch 15 white works with the protective case, tight fit, but ok. I never tried with the bumper, can do it later. Which color did you buy? I made the experience, that they are different in size, black seems to be the widest version.
I bought black 
Then it’s maybe just luck, which one is bigger than others 
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I tested now my white Mitch 15 with the bumper instead of the protective case and it fits too.