For 3B with vanishing extensions (after rebooting)
You need a PERSISTENT TCE directory (and optionally /opt, /home, etc.)
Boot into your 3B with PiCore v9.x
The SD card is going to be split into two pieces, Partition's 1 and 2
/mnt/mmcblk0p1 is the DOS/boot partition which for most of us we leave alone unless we have to tweak something, in your case, the command-line text files.
run sudo fdisk /dev/mmcblk0 and if the second partition exists (mmcblk0p2), DELETE it and create a new one (filling the remainder of the card.)
Reboot and format your new partition with mkfs.ext2 /dev/mmcblk0p2 -L tcedisk which also gives the partition a name of "tcedisk"
Either using PiCore or your desktop, you'll need to edit the cmdline[].txt files and add tce=LABEL=tcedisk to the end of the line in order to make your tce (extension) directory a part of your SD card which will be there after you reboot.
* If your SD card is 16GB or higher you might want to use gdisk instead of fdisk (tce-load -wi gdisk) though for the rookie it might be a bit more complicated than fdisk - start off with 8GB or less if it poses and issue.