In order to get the most out of it, you should be using identical drives. The more they are identical, the easier your life will be.
I.e.: 3x 30GB drive from Manufacturer XXX and Model YYY (all same) == 90GB of storage.
You "can't" put a single drive on its own stripe - there's no difference to it being on its own anyway. RAID begins only with 2+ drives. And in general, it's good RAID-etiquette to use identical drives.
Obviously, this is an "ideal world" scenarion and most end users (like you) have not only got different drive-models/manufacturers but also sizes. That's why RAID IMHO is something for ppl who can afford it & put up the effort to do it properly.
Preferrably, (in case it needs mentioning) the drives should be at the same speed/transfer rates & all that. I.e.: 7200 rpm, UDMA/100 & so on.
You probably can do your RAID with those two, but I wouldn't be surprised if you run into a few unpleasant effects as a result.
Why do you want it anyway - do you do anything that really requires RAID-0? Do be aware that if a single drive of a RAID-0 fails, the whole array fails. So make sure you do regular backups. Otherwise, don't even consider RAID-0.
Hope this helps somewhat

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