https://www.amazon.com/USB-C-USB-Windows-Transfer-Cable/dp/B...
which is insanely fast (much more than SFTP over Ethernet) but I haven't seen one with USB-C plugs.
https://www.quora.com/Can-you-transfer-files-from-one-laptop...
but I don't know if I believe it. I remember back in the day you could boot a Mac into a SCSI target mode and access the hard drive with Firewire, I dunno if you can do it today with Thunderbolt.
Or if you really need USB3 for achieving the maximum speed possible - I would like to find an external 2GB disk with USB3 wiring.
If at least one of your PCs is a proper computer (not laptop/mini-PC with almost all of PCI interfaces just stubbed) having at least 1 free SATA slot then just take the disk from one PC and connect it to another.
Would just connecting them via Ethernet work, or would I need to hook them into my router.