FAT32 doesn't somehow magically force the card back to UHS-1/Class10 speeds. You can use FAT32 with UHS-3 cards at UHS-3 speeds just fine, as long as you have a UHS-3 reader. (UHS-3 adds an extra row of pins to the card.)So yeah, if you FAT32, you lose UHS and it reverts back to SD speeds of Class 10. So don't pay extra for UHS-3 mSD cards for your 3DS. UHS mostly applies to exFAT and NTFS speeds.
3DS, of course, doesn't have a UHS-3 reader, so UHS-3 cards are a waste of money if you're getting an SD card that will only be used on 3DS.
Also, once again, I should remind everyone that "Class 10" refers to minimum guaranteed write speeds, not maximum speed, and not read speed. It also reflects the speed of the card, not the reader; I don't think the 3DS's SD hardware can even hit 5 MB/s.






