Idiot -- he made a RAID 0 array
Upon reaching ~0.9 TB -- Remember that number, commentards: That's 320 GB x 3 -- the storage space vanished! Yes, vanished.
After invoking the Storage Space Manager, I discovered that the space was deemed “full” and that I was to add another disc. I also took a look at the volumes itself. Hmmh. The 320 GB disc was 100% filled, the 1 TB was at 32% and the 2 TB at 16%. And what are 32% of 1 TB and 16% of 2 TB? Why, 320 GB!
So, instead of creating a Parity storage space, it simply downsized every hard disc to the lowest denominator, i.e. 320 GB.
The idiot created a RAID 0 array, and a RAID 0 array will pick the smallest disk in the array to base the size of the array from. This was like the software RAID 0 capability that was included in Windows since Windows NT 3.5. At least in NT 3.5 the remainders of the other disks would still be available, and (having not looked at Storage Spaces on Win 8 yet) I'll guess that the remainders of the other disks are still available for other uses.
A RAID 0 array is not the same as an extended volume, which is not dependent on the sizes of the disks. That was introduced in Windows 2000 with dynamic disk partitioning.
A parity storage array would be at least RAID 5, also known as "striping with parity." And that would be 640 GB with three 320 GB disks, not 0.9 TB.