What are the max. simple and open file sync solutions without web interface?

Question

linux

There is a file and there are 3 laptops. They are not at home with laptops, someone always drags them around.
The file can be changed on any of them.
A person with a laptop may be on a business trip. For six months to a year. Also, he often may not have access to a router, or he may only have wifi itself, or mobile Internet.
I have a vps server, if that.
So how do you sync a file? Like rsync or syncthing? Which is better, which suits me? Or is there something else?
syncthing requires physical access to the router in order to install there? If so, then often I may not have access - it will not work.
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Dropbox, nextcloud, Yandex disks, etc. - not suitable. We need a solution that is as open and close to Linux as possible.
Web interface is optional.

Answer 1

lsyncd