devtmpfs 118672 0 118672 0% /dev
A filesystem that contains information about devices. The files is pointers to device drivers in the kernel.
tmpfs 25388 (8044) 17344 32% /run
Used by daemons to save temporary information.
tmpfs 5120 0 5120 0% /run/lock
Used to prevent the same daemon to be started more than once.
tmpfs 50760 0 50760 0% /run/shm
Can be used by You to save temporary files instead of /tmp.
if the tmpfs dirs are on devtmpfs,
devtmpfs - 3 subdir tmpfs: 35MB missing
or are these not related?
No, they are like different partitions on a hard disk.
tmpfs is used to minimize wear on the SD-card. It takes a portion of RAM memory and is always cleared when the computer shuts down, so no good to save lot of data.