colico - When you say you've "added all devices s/n", did you also go to the "Device Assignments" screen in Apple Business Manager and assign those three serial numbers to the MDM server (see below). If that completes successfully, then you should be able to sync devices in Workspace ONE and see those devices in the Enrollment Status page.
The next thing to check would be the "Default Profile" assigned to newly synced devices. Under Settings > Devices & Users > Apple > Device Enrollment Program, check that you do not have None selected as your default profile assigned to newly synced devices. Without a DEP profile assigned to the device, it will not bring up the organization settings during the Setup Assistant. If the device has already synced into Workspace ONE while the default profile was set to none, you'll need to select the device(s) in the Lifecycle page, click More Actions, and then click Assign Profile. You'll be able to then choose the DEP profile to set for the device. Refresh the Enrollment Status page, change the layout to "Custom" and then make sure the "Profile Status" field shows "Assigned."
Finally, if things still don't seem to be working, go back to the Device Enrollment Program settings page in Workspace ONE and click the Renew button. In Apple Business Manager, you'll have to go to Settings and then find your MDM Server and click Download Token. Immediately upload that token into the "renew" screen in Workspace ONE. Then, retry the "Sync Devices" button.
