Syncing bookmarks/history/etc. between multiple (3+) devices

Hello, I've recently finished building a new computer and have been working on setting everything up, and Firefox Sync has made restoring my browser info quite easy. However I've got two other computers I'd like to try and transfer data from, and unfortunately made the mistake of not syncing the browsers between those two computers beforehand. So I'm wondering if it's possible to sync the data between the browsers on all three computers and how to do so, and if I attempt to sync the data between them, will they merge together or will one overwrite the other? Thank you for your time and assistance.