Using “GET” variables might be a slick way to go. Any time you see something like “?x=y” in the site’s url, it’s able to pass information to the site that you can then use for different things.
In your index file, try writing something like this:
<?php if($_GET['site'] == 1) { ?>
Hey, welcome to site 1!
<?php } else if ($_GET['site'] == 2) { ?>
Hey, welcome to site 2!
<?php } else { ?>
<a href="https://www.yoursiteaddress.com?site=1">Go to site 1</a>
<a href="https://www.yoursiteaddress.com?site=2">Go to site 2</a>
<?php }; ?>
Basically that tells the browser that if the address contains ?site=1 to say “Hey, welcome to site 1”, if it contains ?site=2, say “Hey, welcome to site 2”, and otherwise to give links to the 2 different sites.
There’s plenty of different ways to do it, but the benefit is that you can manage all three of the different home pages within your index page.