Presently I have put them in following dir like:
/var/www/wordpress1
/var/www/wordpress2
/var/www/web3

Now how to Set Up Nginx Server Blocks? - it’s only here I am strucked :(

You have to set the root to those directories on each block.

I’ve given a very bare config below:

In /etc/nginx/sites-available/wordpress1

server {
  listen 80;
  listen [::]:80;

  root /var/www/wordpress1;

  server_name wordpress1.tld;
}

In /etc/nginx/sites-available/wordpress2

server {
  listen 80;
  listen [::]:80;

  root /var/www/wordpress2;

  server_name wordpress2.tld;
}

In /etc/nginx/sites-available/web3

server {
  listen 80;
  listen [::]:80;

  root /var/www/web3;

  server_name web3.tld;
}

Now to activate those sites, just run ln -s /etc/nginx/sites-available/sitename /etc/nginx/sites-enabled/ then restart Nginx.

As sites-available/ and sites-enabled/ dir were not there so I created them manually and after that I
followed all your mentioned steps.

And then configured DNS also. It propagated and reflected the IP changes.
But when hitting the site, its showing me the Nginx deafult page instead of wordpress installation page.

Please note that i have installed LEMP using this tutorial : Link

Please help.

Ah. My bad. I missed the OS you were running. Those directories are on Debain-based systems. It seems for CentOS /etc/nginx/conf.d/ is where your configs are. You will want to make your configs there.