Here’s an idea of what I think you could do if I understand the problem correctly:
<div id="primary" class="content-area">
<main id="main" class="site-main" role="main">
<div class=" entry-content">
<?php
if ( have_posts() ) {
$post = $posts[0];
$c = 0;
$i = 0;
while ( have_posts() ) {
the_post();
// start a new row for the first post and every three posts
if ( ! $paged && ( 0 == $c || 0 == ( $c % 3 ) ) ) {
get_template_part( 'template-parts/content', 'aktuelles-first' );
echo my_code_open_row();
} else {
get_template_part( 'template-parts/content', 'aktuelles' );
}
// close the opened divs above
if ( $i > 2 ) {
echo my_code_close_row();
$i = 0; // Reset the row close counter
}
$c++;
$i++;
}
// Close the row if we never get to the row closing
// before we leave the loop
if ( $i < 2 ) {
echo my_code_close_row();
}
}
?>
</div>
</div>
</main><!-- #main -->
</div><!-- #primary -->
<?php
function my_code_open_row() {
return <<<HTML
<div class="accordion">
<div class="row">
HTML;
}
function my_code_close_row() {
return <<<HTML
</div>
</div>
HTML;
}