Yes, 3Gb/s SATA equal SATA 2
Longer answer with some background:
Currently there are three SATA standards:
SATA 1, aka SATA 1.5GB/sec
SATA 2, aka SATA 3.0GB/sec
SATA 3, aka SATA 6.0GB/sec
In theory all devices are compatible. A SATA-2 drive on a SATA-1 controller should just work, albeit at a maximum transfer speed of about 130MB/sec. ( SATA-1 speed, and compensating for the overhead in the protocol and the encoding).
A single harddisk is not that fast and SATA-1 is still more than adequate for regular desktop usage.
Most SSDs are a lot faster and will be limited by SATA-1, for those you really want SATA-2 or 3. Another use for faster SATA speeds is with port multipliers and several harddiks on a single SATA link.
But, for normal desktop use: Everything should be interchangeable. A few are not and most of those have jumpers to fall back to SATA-1. (Essentially this means the firmware on the HDD is not up to snuff. It should not be needed).