Before going further ... a question ... are these 3 CentOS 7 servers in a virtual environment .. ie, VMWare then CentOS 7's as guest OS's in the VMWare environment? or some other virtualized system? (uhhhh ... they appear to be just nodes on the same subnet, BTW).
Can see something with the setup you might need to re-consider - moodledata on same server as DB (from first hand experience in assisting an entity whose moodle admin thought he had 'unlimited space') especially IF that 1TB drive isn't partitioned ... partitions: typical / where the databases live in /var/lib/mysql ... consider making a partition /data (the 1TB) /data where moodledata (for filedir really) would reside.
Reason .... 1 parition could be filled up (yep, doesn't take long) on a 1TB IF someone (a Moodle Admin not really knowing) set autobackups with 'keep all' or if one gets a teacher with a large course (say in to 50+ Gig range) did full course backups once a week and never cleaned up/removed older backups .
The 1TB drive could fill up leaving no space for DB means DB corruption .... can't launch MySQL server.
The lions share space in moodledata will be filedir and I have seen servers setup where on the Web/Apps server /var/www/moodledata uses a mount point only for/to filedir.
The other directories found in moodledata ... cache localcache environment lang lock muc repository (if used) sessions temp trashdir remain in /var/www/moodledata/ (faster access).
It would be important how one sets up that mount point so review the options in setting up a mount point in fstab.
Do NOT mix Windows networking ... Samba ... in the mix of what you have ... get that out of your mind.
One of the reasons you experienced 'loss of mount' could have been how one mounted - if one didn't enter the mount in fstab but, rather, had just used a command to mount ... when the apps server or in your case the DB box was rebooted, the apps server didn't know to remount.
Be careful with settings for the mount in fstab ... on virtualized environments ... like Google Compute Engine ... if the mount is permanent and the fstab settings are in-correct, the server won't boot .... gets to the point of mounting, can' t find/do the mount ... halts the bootup.
'spirit of sharing', Ken