I think the internal storage formatting is probably the culprit, I agree with @swhcz. Forum topic and explanation about this problem here.
You can also have a look at the #sdcardguide.
I don’t think storing apps on an external storage is possible (I may be mistaken though). But do you really need to store apps on the sd card? Aren’t the 64GB of the FP3 enough. I can understand you want to store the data of your apps on the sd card, because this can eat up quite a bit of space, but you really need to have your apps on your sd card?