Thanks so much for responding!
As a Windows guy, I'm actually helping a musician friend. He has 2 old Mac Pros, bought this early 2009 running OS 10.12.6. I added 15 GIG ram, USB 3 PCIe card, 1 TB HD, 6 TB backup ext. HD.
He runs Pro Tools and Logic Pro with UA Apllo Firewire, just bought Thunderbolt 3 card for Apollo .
He learned about Universal Audio Luna, only for Thunderbolt 3, wants to abandon PT and L Pro for Luna, asked me to upgrade again.
You confirmed what I guessed, that this motherboard does not support Thunderbolt 3, even with PCIe card.
I would assume a new motherboard is the only answer to salvaging his investment in the old Mac Pro.
After raising a family, pushing his music aside, he's desperately trying to start back up, with minimum cash.
Excuse if this is more info than you wanted to know!
BTW, with this tower running 10.12.6, everything pointed to it supporting 10.13.2 with Thunderbolt 3.
I appreciate any advice.