**Step 1: Create the Linux User** - Create a new user on the Linux system using the `adduser` or `useradd` command:  ```bash  sudo adduser admin_user  ```  Replace "admin_user" with the desired username for the admin user. **Step 2: Assign Database Access** - For each database, you'll need to grant the admin user appropriate privileges. Here's a high-level overview for three common databases:  **MySQL/MariaDB:**  - Access the MySQL shell as a privileged user:   ```bash   sudo mysql   ```  - Create a new user and grant privileges:   ```sql   CREATE USER 'admin_user'@'localhost' IDENTIFIED BY 'password';   GRANT ALL PRIVILEGES ON database1.* TO 'admin_user'@'localhost';   GRANT ALL PRIVILEGES ON database2.* TO 'admin_user'@'localhost';   GRANT ALL PRIVILEGES ON database3.* TO 'admin_user'@'localhost';   FLUSH PRIVILEGES;   ```  **PostgreSQL:**  - Access the PostgreSQL prompt as a privileged user:   ```bash   sudo -u postgres psql   ```  - Create a new user and grant privileges:   ```sql   CREATE USER admin_user WITH PASSWORD 'password';   GRANT ALL PRIVILEGES ON DATABASE database1 TO admin_user;   GRANT ALL PRIVILEGES ON DATABASE database2 TO admin_user;   GRANT ALL PRIVILEGES ON DATABASE database3 TO admin_user;   ```  **MongoDB:**  - Access the MongoDB shell:   ```bash   mongo   ```  - Switch to the admin database and create a user with appropriate privileges:   ```javascript   use admin   db.createUser(    {    user: "admin_user",    pwd: "password",    roles: [ { role: "readWrite", db: "database1" }, { role: "readWrite", db: "database2" }, { role: "readWrite", db: "database3" } ]    }   )   ``` **Step 3: Test Database Access** - After setting up the database access, you can test the admin user's access to the databases using the respective database clients or command-line tools. By following these steps, you can create an admin user on a Linux system with access to three different databases. Be sure to replace "admin_user" and "password" with your desired username and password, and adjust the database names and privileges to match your specific environment.