HDDs can be SATA 3 (6Gb/s) "compatible", not SATA 3 (6Gb/s) "capable".
No HDD can spin fast enough to achieve 6Gb/s speeds. The ST31000524AS has a 32MB cache buffer that can transfer its contents at 6Gb/s speeds, but that's it.
The Read/Write speeds for the drive are up to 125MB/s. http://www.seagate.com/docs/pdf/datasheet/disc/barracuda-7200-12-ds1668-6-1101us.pdf
So you can connect the drive to a 3Gb/s port or a 6Gb/s port and you will get the same performance.