Simply Step-by-step guide
1. insert SD card to SD slot of v3 router
2. run a command "dmesg"
3. output from "dmesg" command looks like below
. . . mmc1: host does not support reading read-only switch. assuming write-enable. mmc1: new high speed SDHC card at address 1234 mmcblk0: mmc1:1234 SA04G 3.68 GiB mmcblk0: p1
4. device of SD card is mmcblk0: p1
5. mountig SD
e.g. mount /dev/mmcblk0p1 /mnt