Multi app authentication

We are building a platform and currently have 3 apps.

App 1 is a marketing app, app 2 an e-commerce app and app 3 an account/subscription management app. Each app has their own laravel api and their own nextjs frontend.

Now we want a single user account for all 3 apps, so the user can login and can make calls to all 3 api's. My question is : where would we build the authentication system? should we build a seperate service for the authentication or should we build it in app 3 since that is where the account management & subscription information is located? and also, we need to be able to access the same user information in all 3 apis