Problem with shortcode

I got this code to show my thumbnails images and videos on my blog page, but when i try to use to show the thumbs in another page, it dont work.

Function to display images and videos thumbs:

function show_blog_posts() {
global $post, $posts;
$first_vid = '';
ob_start();
ob_end_clean();
$output = preg_match_all('/<iframe.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
$first_vid = $matches [1] [0];

if ($output == '1') {
    echo '<iframe width="100%" height="400" src="';
    echo $first_vid;
    echo '" frameborder="0" allowfullscreen></iframe>';
} else { ?>
            <?php the_post_thumbnail('large');?>

 <?php }

My shortcode

add_shortcode('recent_blog_posts', 'shortcode_recent_blog_posts');

function shortcode_recent_blog_posts($atts) {

        $str = '';
        $args = array( 'numberposts' => 3, 'post_status'=>"publish",'post_type'=>"post",'orderby'=>"post_date");
        $postslist = get_posts( $args );

        foreach ($postslist as $post) : setup_postdata($post);
            $str .= '<div id ="article-section-box2" class="span3">';
                            $str .= '<div id= "image1" class="image-arcticle">';
                            $str .= show_blog_posts();
                            $str .= '</div>';
                            $str .='<p class="article_text">'.excerpt(18).'</p>';
                            $str .='</div>';
        endforeach; 

        return $str;

Is there anything wrong? Thanks