As @Vegard mentioned, we need more information to give a complete answer. However, based on my understanding, it sounds like you want to make your Auth service act as an OIDC provider and mirror users in App1, App2, and App3.

In that case, you can use the id_token issued by your OIDC provider to authenticate users across your various applications.

I've implemented a similar setup in this repository: django-oidc-provider – it might help as a reference.