Then you need to create 4 projects.
1 project for the login/Single Sign On (SSO). This project will hold your users table and an API for authentication (JWT/Passport/etc.).
Then from any project, when user submits users details, you send them to the SSO project via API and gets a response if the login was successful. The SSO should return a token to the project from which the user is logging in.
You can learn more about Laravel Passport and token grants here: https://laravel.com/docs/5.4/passport
You mentioned using JWT, here are some resources to get you started:
https://dzone.com/articles/from-json-web-token-to-single-sign-on-part-1-creat-1
Single sign-on flow using JWT for cross domain authentication
Good luck mate!