I'm assuming your talking about the thing that USED to happen with IDE where if you put an optical drive on the same cable as a hard drive the drive would slow down to ATA33 just like the optical disk. I'd like to point out that this isn't true anymore for IDE drives as well seeing as they developed independent device timing shortly after this problem was discovered.
As already mentioned, this has never been the case with SATA as each device is on its own cable. You have nothing to worry about. And don't worry when you fail to see your drives not hit the limit of "SATA 3". Thats the max transfer speed of the interface, the drives themselves can't go that fast.http://www.storagereview.com/guide/confTiming.html