firewire 800 to thunderbolt 3

The only way I know to do that is to use two adapters, both available from Apple. The FireWire 800 cable first goes to a firewire800-to-thunderbolt2 adapter, then the thunderbolt2 end of that goes into a TB2-to-TB3 adapter. That works as I’ve used that with a FireWire 800 OWC hard drive and my 2018 Mac mini.

if you have any FireWire 400 devices, just add a firwire400-to-firewire800 adapter at the front if that sequence (that too works with my Mac mini’s TB3/usb-c ports).

https://www.apple.com/shop/product/MD464LL/A/apple-thunderbolt-to-firewire-adapter

https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter?fnode=8b