A USB enclosure for SATA 3 is utterly pointless; the only advantage SATA 3 has over SATA 2 is the same advantage SATA 2 had over SATA one; higher throughput. Most hard drives cannot even come close to saturating a normal SATA I connection, let alone SATA 2 or SATA 3- and even if they could, USB2 is limited to 480mbps anyway so even SATA I would be bottlenecked.
All SATA drives can be placed in all SATA connectors. a SATA 3 drive will work with a SATA 1 interface; a SATA 1 drive will work with a SATA 3 interface, etc.