ThunderBolt 3 to Firewire 800?
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.