How do I use MacOS Terminal or other app to format an MBR FAT32 SD card as a 0x0C partition, rather that 0x0B?
Your fdisk invocation should have already partitioned it as type 12. You can press the Ignore button and verify via Terminal that that's the case, then simply eject the disk from your computer and use it wherever you intend to use it.
If you intend to use this drive on your Mac, you will need to format and mount it after running fdisk:
sudo newfs_msdos /dev/disk3s1
sudo mkdir /Volumes/FLASH
sudo mount -t msdos /dev/disk3s1 /Volumes/FLASH

Here FLASH is an arbitrary volume name that I chose for sample purposes; feel free to choose your own.
As a clarifying point, the s1 portion of /dev/disk3s1 targets the first partition of disk3. If you were to specify just disk3 instead, the newfs_msdos command would overwrite the partition table.