Finally AT&T AllAccess included in the Microsoft Surface 3 Windows 10 recovery image worked for me.  It's 4.1.0.xxx, which I never saw available anywhere else.   Some notes:

1. The problem of resetting using a USB recovery image was resolved (I tried both Windows 8/10 images) after I saw this helpful link:

https://answers.microsoft.com/en-us/surface/forum/surfpro2-surfupdate/reinstall-windows-881-using-the-recovery-image/229db212-37b6-4577-bd12-2aea9c680fc3  

There was some variation between Windows 8/10, esp. in step 11, and something unexpected in step 13 that appeared risky and serious.  I didn't document those as I was trying different things quickly.  I took the best guess as to a safe next step... knowing I could always fall back on an OS install USB that I had prepared and proved working.

2. Toward the end of resetting with this Microsoft image, I skipped sim/WiFi setup.  After the completion and Windows was on, I inserted my T-Mobile sim (without any pin attached/saved), searched for and opened AT&T AllAccess app in windows.   There then appeared a prompt for unlock code.  I am not sure about the app's behavior if done a different way.

Thank you!
The stock AT&T Windows 10 recovery image from Microsoft's download page prompted for the unlock code. It was asking for a 10 digit code, but the code I had was approximately twice that long. I input the full code I received anyway and it worked immediately.

There was no indication that it was successfully unlocked, but it was able to connect to the network afterward.

Can you help me pls???? I download the default image from the webpage of microsoft for the surface 3 AT&T, but dont include the program AT&T All Access and if I install from the webpage don't work, please some advice, i download from here the image

https://support.microsoft.com/en-us/surfacerecoveryimage

Then i put the model and serie of the surface and send me this "Surface 3 LTE - 64GB 2GB (AT&T) - Windows 10 Version 1511", download and install but don't have the program(AT&T AC) and i can unlock my surface, i tried everything and nothing, PLSSSSSSS!!!!!!!!!!!

Since the downloaded image (applicable to the specific LTE tablet) worked for me and someone else after me, I would suggest checking the following:

1. whether the restore from the image is successful (probably by checking the windows 10 version and updates and presence/absence of applications)

2. search for AT&T AllAccess or anything associated with AT&T in windows 10

3. whether Microsoft has since replaced the image with one that does not include AT&T AllAccess.  This is just to find out why you don't see AT&T AllAccess.

hello 

I have also same problem  like above.

my window is 11 and at&t version 10.3.100.2

but I have not seen

not prompted to enter device unlock code area. 

let me which isthe best way to unlock my surface 3.

help me please.

I have no At&t sim and its data.

because i buy used from amazon.