Yep you'll have to reinstall your OS on the raid.
As to how much better 3 drives would be than 2, I'm not sure. Basically in a RAID0 setup, the data is broken up and distributed to all the drives. So theoretically, as you add more and more drives to the setup, the percentage of speed increase at which the files are written should decrease.
edit: so yes, 3 drives should be better than 2.
edit edit: and yes, that drive looks fine.
edit edit edit: no, not exponentially. My mind was elsewhere
edit edit edit edit: and again, it's not totally correct to just multiply it either. But for the number of drives you're dealing with, yeah it's close enough I guess
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