server {  
    listen 80;
    server_name example1.com;
    location / {
        proxy_pass http://localhost:3000;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;
    }
}


server {  
    listen 80;
    server_name example2.com;
    location / {
        proxy_pass http://localhost:8080;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;
    }
}

Error Log :
2020/03/17 14:13:44 [error] 1880#1880: *88 connect() failed (111: Connection refused) while connecting to upstream, client: 157.37.238.120, server: example1.com, request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:3000/", host: "www.example1.com"
2020/03/17 14:13:44 [error] 1880#1880: *88 connect() failed (111: Connection refused) while connecting to upstream, client: 157.37.238.120, server: example.com, request: "GET /favicon.ico HTTP/1.1", upstream: "http://127.0.0.1:3000/favicon.ico", host: "www.example1.com", referrer: "http://www.example1.com/"