Your link displays the Intel IRST support page, and no information about your SATA controller. It looks like you may have one of the newer Intel 6 series chipsets, but all models do not have SATA 6Gb/s support, so I'm not sure if yours is one of them. What is the model number of your mother board or PC?
IRST will show the actual negotiated SATA transfer rate between the controller and drive, in your case 3Gb/s. So either your Seagate HDD is connected to a SATA 3Gb/s port instead of a SATA 6Gb/s port, your SATA chipset does not have SATA 6Gb/s support, or in some cases (laptops), the manufacture did not use the SATA 6Gb/s capability of the SATA controller, assuming the controller provides that support.
Unfortunately, while standard HDDs may support the SATA 6Gb/s protocol, NONE of them will even reach the maximum speed that SATA 3Gb/s support will provide. Frankly, my (so called) SATA 6Gb/s HDD has a sequential read speed right at the real world limit of the SATA 1.5Gb/s protocol, or ~130MB/s when I tested it. SATA 3Gb/s provides a real worlds sequential read speed of ~260MB/s. Note that Seagate's specs for the best of their SATA 6Gb/s drives are a maximum of 210 MB/s, and an average rate of 156MB/s:
In the real world, simple users cannot tell the diference between the two specs when performing general tasks. At most, the performance improvements can be noticed when copying a large size file from one drive to another.