Hi,
There is some information about backup/restore in the operations guide (NSX-T 3.0 Operation Guide - VMware Technology Network VMTN)
This involves using the backup function on the manager to save it to a remote server.
Assuming in a production environment having 3 managers, if one completely dies, you can install a new one and restore the config on it.
You will have to backup your NSX appliances using a file based backup.
I suggest you read this post: A step by step how to Backup and Restore NSX-T v3.0 > ProVirtualzone - Virtual Infrastructures
For that you will need a SFTP share.
If that share resides in a VM you can backup that VM with Commvault.
Also, if you are not doing so, I suggest you use the same approach for your vCenter Server Appliance