When adding a new device in the V3 Console it is currently not possible to generate a new DevEUI.
(The DevEUI is actually not generated but will be issued from a valid range of DevEUI.)
The DevEUI is required for adding a new device to a V3 application.
Existing V2 devices will already have a DevEUI but many new (DIY) devices will not.
As workaround a temporary device can be created in the V2 console and then copy the DevEUI for use in V3. The temporary device can then be deleted.
New registrations on V2 will be disabled starting June 30, 2021.
This means that the workaround for creating DevEUI on V2 will no longer be available.
It currently is still not possible to generate a DevEUI in the V3 console.
A release date for when this will be available has not yet been announced.