Why won't an fsck-declared-good volume mount?

I had same problem... fix/work-around was in Terminal do something like:

mkdir /tmp/3

sudo /sbin/mount_hfs /dev/disk3s3 /tmp/3

df -h  /tmp/3
Filesystem     Size   Used  Avail Capacity iused      ifree %iused  

/dev/disk3s3  464Gi  249Gi  215Gi    54% 3279960 4291687319    0%   /private/tmp/3

BTW, if you have extra ram, you can do faster checks of volumes, esp Time Machine disk, with a command like:

sudo fsck_hfs -f -c 6200m /dev/disk3s3 

** /dev/rdisk3s3 Executing fsck_hfs (version hfs-366.70.1). ** Checking Journaled HFS Plus volume. The volume name is SeaTMimac1012.noindex ** Checking extents overflow file. ** Checking catalog file. ** Checking multi-linked files. ** Checking catalog hierarchy. ** Checking extended attributes file. ** Checking multi-linked directories. ** Checking volume bitmap. ** Checking volume information. ** The volume SeaTMimac1012.noindex appears to be OK.

Tho it was ok, none of these would mount it: DiskUtility (horrible one in 10.12), diskutil mount, etc.