I received my 6th or 7th Surface 3 a few weeks ago. Booted in Windows 8.1 (which I still love by the way!) Went to Microsoft.com, searched for some "upgrade to Fall Creator's Update" and installed. Works like a charm!
I've since discovered this MS Answers article that seems to say Win 10 is automagically delivered to Win 7 and 8.1 users as part of the normal Windows update process. I guess that means it'll be hard NOT to have Win 10 if you've got auto-update turned on.