Haven't tried a single 3TB drive, but 2 seems to work pretty well, but with some quirks...
Recently bought a dns-320 and two dirt cheap 3TB WD30EZRX (WD Green Caviar).
Anticipated lots of problems since the Advanced Format drives are incompatible with just about everything i can think of.
Formatted the drives as one mirrored partition, which seemed ok, but Vista32 and my old TViX 4100 could only see it as a 750G drive.
Figured it could be some problems with 32-bit clients, so tried again with 2199GB mirror + the rest as JBOD, and it seems to work ok!
My TViX gets confused about large file sizes via samba mounts, but NFS seems ok so far.
Still wondering if one 3TB partition is possible somehow... Perhaps in the next firmware release?