How to show posts inside echo?

I want to display my posts in another layout on my front page than on my other post pages (/page/2, /page/3, etc.). I found the following code to check if the page is the front page:

<?php
if( $_SERVER['REQUEST_URI'] == '/nieuws/' ) {
    echo ('<h3>home</h3>');
} else {
    echo ('<h3>page</h3>');
} ?>

This works, but when I put the code to display my posts inside the first echo, it doesn’t work anymore.

The code to display my posts looks like this (and works well):

<?php $postnum = 0; if (have_posts()) : ?>
<?php while (have_posts()) : $postnum = $postnum + 1; the_post(); ?>
<?php if ($postnum == 1 ) { ?>
CODE FOR POST HERE
<?php } ?>

<?php if ($postnum == 2 ) { ?>
CODE FOR POST HERE
<?php } ?>

<?php if ($postnum >= 3 ) { ?>
CODE FOR POST HERE
<?php } ?>
<?php endwhile; ?>
<?php else : ?>
<?php endif; ?>

Can somebody help me please?