Will SATA 2 HDD work on SATA 3 MOBO ?

Not only YES, but in-fact there is Almost zero performance difference.
.. The ONLY gain for SATA III HDD on SATA III is in the very short Burst speed. Sustained throughput does NOT saturate a SATA II port. More a marketing ploy - Would wants to refer to their HDD as a OLD SATA II. Almost a waste of a GOOD sata III port for a SSD - LOLs.

.. Not quite true for Hybred HDDs that have a internal 8 gig SLC SSD inside, But still not a real biggy as the Gain is primarily limited to the Loading of the OS during power on.