Well.. I think it depends on how you have your drives setup/attached, as well as how your software is configured. So think about this....
I have all of my rigs essentially setup the same when it comes to the hard drives. Raid 0, then chunks attached to vm's as datastores. So basically, you have raid 0, which pulls from multiple drives.
So my read speads for the small one is 130mbp/s
the larger miner I have is running at 680mbp/s. You shouldn't be running that low with 3 drives. It should look more like 90mbp/s for you, or at the very least, 58-60mbp/s