p2pcopy https://github.com/psantosl/p2pcopy
pcp https://github.com/dennis-tra/pcp
wormhole-william https://github.com/psanford/wormhole-william
p2pcopy https://github.com/psantosl/p2pcopy
pcp https://github.com/dennis-tra/pcp
wormhole-william https://github.com/psanford/wormhole-william
Typically, a firewall allows outbound connections without needing an explicit entry for the protocol, and in the case of magic wormhole, both sides are an outbound connection. So it passes right through.
If you've got security-minded folk managing that sort of thing for you, it's possible that magic wormhole will upset them for this reason. More for policy/compliance reasons than actual security ones.
Yup it's what I do, that 3rd computer having a fixed IP. Conveniently that computer can also keep a copy of the file(s).
Linux/BSDs/OS X (which is kinda a Unx too) all come stock with scp* and I don't really use Windows, so I'm a happy camper.
If I tried to actually come up with the actual commands for this, I'm sure I'd burn a whole afternoon on fiddling with it.