This would all depend on budget and the importancey of the data.
If its mission critical and budget is flexible then I would go for the following:
Copy 1:
A 5 bay NAS unit filled with 3TB drives in RAID. Should give around 12TB of useable space after RAID5 configuration.
Data is accessible remotely if need be.
Copy 2:
5 Bay Expansion Unit connected directly to the above.
Again filled with 5x 3TB drives in RAID5.
Connected directly via eSATA.
Backup schedule set so backup runs frequently to keep a local copy of all the data.
See my YouTube videos and search for the Synology DX510 for a demo.
Copy 3:
This needs to be remote in case the above are destroyed by fire etc.
Another 5 Bay NAS - same unit as in Copy 1.
Same setup as Copy 1.
NAS from copy 1 set to automatically backup to the remote NAS.
This keeps a copy of all data in a seperate location.
The above is all easily possible using Synology (also probably QNAP) units.
Again, it's not cheap but you didn't give an idea of budget.
I've covered a fair bit of this setup in my YouTube channel if you wanted some more ideas etc
