If you've opted in to the Get Windows 10 reservation applet, check to see if the following hidden folder exists on your Surface: C:\$Windows.~BT Looks like the installation media is being pre-cached ahead of installations beginning tomorrow.
Opted in a while back here and am also a Windows Insider member, although ran the technical previews inside virtual machines, not on my SP3. Same Microsoft account used on my SP3 as well as for the Windows Insider stuff.
If you notice that your free space on your drive on your Surface Pro 1, 2, 3 or Surface 3 just got smaller (running Windows 8), don't worry. Windows 10 reservation app, if you have reserved the coming up OS, has started to download Windows 10. It also include a large day 1 update, so the size of the download is bigger than anticipated.
If you see: C:\$windows.~BT, don't delete it! It is Windows 10 setup.
The distribution of Windows 10 upgrade are done in waves. You can get it today, or tomorrow, or up to, if you are unlucky, August.
I'm not sure Winbeta is correct that KB3074683 is the Day 1 patch. First, that one installed on my machines yesterday, and second, it wasn't terribly large (which I know, because it downloaded and installed quickly on my Dell Venue 8 Pro, which is pretty slow at updating). I'm expecting the Day 1 patch to be a much more significant update, and to definitely ship tomorrow.
Edit: Actually, I think I'm wrong. From another report it seems like it might be the Day 1 patch. Sure didn't seem like it to me.