The SATA drives are not a problem on your MacPro 3,1, which has SATA ports available on the logic board, and can be used to add an SATA optical drive, if you want All you need to do is run a couple of SATA cables from there, up to the optical drive.
The SATA cables that you need are included with the drive from OWC that I linked above.
No adapter needed, just SATA cables.