I’m sorry to report that the 2010 Mac Pro motherboard still has a maximum bandwidth of around 690MB/sec for internal SATA ports. That means that just two of the OWC Mercury Extreme Pro RE SSDs almost max-out that Mac Pro SATA bus, and three of them fully saturate it.
The actual speed figures are below the true potential, because the Mac Pro SATA ports share a maximum bandwidth limitation.
Since two SSDs are good for about 510MB/sec reads and 470MB/sec writes (as actually measured), the improvement with three of them is not as impressive as one might hope. Still, there’s no denying that even two SSDs offer a level of performance that outstrips the ability of most programs to use it.