SATA 1 (1.5Gb/s) data transfer speeds are from 1MB/s to 150MB/s.
SATA 2 (3Gb/s) data transfer speeds are from 151MB/s to 300MB/s.
SATA 3 (6Gb/s) data transfer speeds are from 301MB/s to 600MB/s.
The data transfer rate for your drive is 126
Hard drives can be SATA 3 "compatible", but not SATA 3 "capable".
Your hard drive has a 32MB cache buffer which can briefly (milliseconds) transfer it’s contents at SATA 3 speeds.
Hard drive manufacturers label their drives SATA 3 mainly for marketing hype.
Here’s the link to the spec sheet of your drive: http://www.wdc.com/wdproducts/library/SpecSheet/ENG/2879-701277.pdf