RAID is not backup. If something corrupts or deletes files, then copies on all disks are immediately destroyed. Also, RAID 0 isn't really RAID--it's AID!
Sounds like you are asking if you can set up RAID 0+1 which is RAID 1 between two 6TB arrays--one consisting of a single drive, and the other consisting of three disks in RAID 0. Only the NAS enclosure manufacturer can tell you if that is possible before just trying it.
Or if you are just asking if it's possible to manually back up the 3 disks in an array to a 6TB disk all in the same enclosure, then yes just about any enclosure allows you to create a single-disk RAID "array" so you could even image/clone between them.