ThunderBolt 3 to Firewire 800?
Solution 1:
Buy a drive caddy for $22.00 (USB3). Take all your old Firewire drives apart pull each disk out, and slip them into your modern caddy. Plug the caddy into USB and pull your data off as needed.
Don't toss the drives. You can use them for backup media etc. through the caddy. Be sure you do a modern reformat on the drives, and run Disk First Aid.
Solution 2:
After a quick search I found out there is a solution:
Have an Apple Thunderbolt 3 to Apple Thunderbolt 2 cable
Then you can have an Apple Thunderbolt 2 to Firewire 800 cable
I do not think there is a way to connect it directly.
Solution 3:
Just take the drives out of their enclosures. They're going to be standard SATA Drives on the inside. You can then just put it into a Thunderbolt or USB 2/3/C enclosure (~$25 online).
Oops, didn't see @Wayfaring-Stranger's post. Yes. That.
Solution 4:
Slightly different -- but the title makes this question comes up in Google for audio gear as well: if you have Firewire gear you can't convert and want to connect, OWC has a Thunderbolt 3 dock with a Firewire port. Noone else has such, AFAIK. https://eshop.macsales.com/shop/docks/owc-thunderbolt-3-dock/firewire