WP_Query & shortcode : Return 3 articles from a category WordPress

The problem is your $contenu variable.

Try:

$contenu .= '<div class="effect-jazz">'.get_the_post_thumbnail().'<div class="jazz-try">
    <div class="h3"><a href="'.get_permalink().'">'.get_the_title().'</a></div>
    <p>'.get_the_resume(60).'</p><!-- THIS FUNCTION NEEDS TO BE CREATED -->
</div>
</div>';

Do not use ; trailers when using inline php, ex <a href="'.get_permalink().'">.

Use the appropriate return functions:

• get_the_post_thumbnail()
• get_permalink()
• get_the_title()
• Create a return version of the_resume() function

Do use the correct operator .= when you wish to append to a variable, rather than overwrite it with each loop iteration.

Do not line break after $variable .= '.