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.
Well I bought the OWC drive and it was not that easy to install. I got everything done except replacing the Optical disk tray to reseat. It just does not want to fit. Too many cables-I don't know but something is preventing it to slide in all the way.
You probably just need to remove the trim bezel on the front edge of the tray. Most are just attached with little tabs, and you need to unlatch the tabs that hold the tray bezel in place.
It's a bit fiddly on some drives.
I guess you mean that the drive carrier (the mount for the optical drives) won't seat into position?
If you got the OWC drive, then you need to follow the video that shows you how everything fits, including how to route the additional cables (assuming that you are changing one of the PATA drives to an SATA drive. If you are keeping both as PATA, then there shouldn't be an issue with "too many cables"
Anyway, should be simple to see what is preventing the seating of the optical drive carrier.
Can you tape extra length out of the way of the carrier? (Duct tape or package tape should be helpful there!)
Another question: Do you have the latch lever up (unlocked), on the back panel, when you try to seat the optical carrier?
I think if you can't seat the carrier, then you also can't move the latch all the way down (?)