Here are the main issues to address in your sample code:

• Both nginx and wordpress Docker images listen on port 80 by default. So you should map wordpress to a different one. For example 8080

• All the images will not be able to see each other unless you set up a network for them.

• Update nginx configuration to remove the port for wordpress. Being in the same network they see each other usin their host names only (so their image name)

• Had to change the way you declare the volumes used by wordpress and mysql images

So this is what I suggest to have:

docker-compose

version: '3.1'

services:
  nginx:
    image: nginx
    ports:
        - 80:80
    volumes: 
        - ./nginx:/etc/nginx/conf.d
    networks:
      - backend

  wordpress:
    image: wordpress
    ports:
        - 8080:80
    restart: always
    environment:
      WORDPRESS_DB_HOST: db
      WORDPRESS_DB_USER: exampleuser
      WORDPRESS_DB_PASSWORD: examplepass
      WORDPRESS_DB_NAME: exampledb
    volumes:
      - wordpress:/var/www/html
    networks:
      - backend

  db:
    image: mysql:5.7
    restart: always
    environment:
      MYSQL_DATABASE: exampledb
      MYSQL_USER: exampleuser
      MYSQL_PASSWORD: examplepass
      MYSQL_RANDOM_ROOT_PASSWORD: '1'
    volumes:
      - db:/var/lib/mysql
    networks:
      - backend

volumes:
  wordpress:
  db:

networks:
  backend:
    driver: bridge

nginx.conf

server {
    listen 80;
    server_name localhost;
    location / {
        proxy_pass http://wordpress/;
        proxy_set_header      X-Real-IP $remote_addr;
        proxy_set_header      X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header      Host $host;

    }
}

You can check more details about networking in Docker Compose in the documentation.