I have been interested in using BitTorrent Sync to synchronize my files. My question is: Is my encrypted torrent data being shared through 3rd party computers or only the computers I register? For example, if I install Sync on 3 machines, will all of that sync'd data only ever stay on those 3 machines and no data - encrypted or not - ever travels to other computers with Sync installed.
While Sync uses BitTorrent technology, it creates swarm only from your machines. Particularly in your case it will create a swarm from your 3 machines, that nobody could join, and will use p2p to effectively send data between them.
PS. I am involved in Sync development.