Yes, use the 3rd Drive as a DV Scratch disk.
Although HFS+ handles Fragmentation ridiculously well, it can still get fragmented if you move large files around on it a lot, and if the two RAID Drives get too full (which can happen real quick with DV) they can be a bit tricky to tidy up.
Also although a Striped RAID Array is much quicker than a single Disk Drive, if the tiniest thing goes wrong with either Drive, you could loose everything on the Array. Using the Array for DV would mean that it gets thrashed more, and is more likely to fail.
Using a scratch disk is the ideal solution, since if it gets too thrashed and crashes, you're only going to loose whatever you just pulled off DV tape anyway, which can be pulled back off tape onto a replacement drive. There's a reason it's called a Scratch disk.
The Technology for Striped RAID only applies to two (or Four, or Eight) Drives. I presume MacOS X uses Striped RAID, since the two physical disks appear as one big volume.
So no, you won't be able to Stripe all Three Drives at once.
The other type of RAID is Mirroring, where both Drives store the same information. It isn't as fast as Striping, and means you effectively halve your Data Storage, but does mean that if one Hard Drive goes, the other still contains all the Data.