$args = array(
    'post_type' => 'post',
    'posts_per_page'        => -1,
    'orderby'               => 'publish_date',
    'order'                 => 'DESC',
    '_shuffle_and_pick'     => 3 // <-- our custom argument
);
$loop = new WP_Query( $args );

add_filter( 'the_posts', function( $posts, WP_Query $query )
{
if( $pick = $query->get( '_shuffle_and_pick' ) )
{
    shuffle( $posts );
    $posts = array_slice( $posts, 0, (int) $pick );
}
return $posts;
}, 10, 2 );