<?php
$loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 3));
while ( $loop->have_posts() ) : $loop->the_post();

$custom = get_post_custom($post->ID);
$product_url = $custom['product_url'];
foreach ($product_url as $key => $value)
echo $key . " => " . $value . "<br />";

$custom = get_post_custom($post->ID);
$case_study_url = $custom['case_study_url'];
foreach ($case_study_url as $key => $value)
echo $key . " => " . $value . "<br />";

?>

// All my mark-up

<?php endwhile; ?>

<?php
$loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 3));
while ( $loop->have_posts() ) : $loop->the_post();

$product_url = get_post_meta($post->ID, 'product_url', true);
$case_study_url = get_post_meta($post->ID, 'case_study_url', true);
echo "Product: $product_url, Case Study: $case_study_url";
?>

// All my mark-up

<?php endwhile; ?>

<?php
$loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 3));
while ( $loop->have_posts() ) : $loop->the_post();

$all_meta = get_post_custom($post->ID);
foreach($all_meta as $meta_key => $meta_value)
    echo "$meta_key: $meta_value<br/>";

endwhile;
?>

<?php
$loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 3));
while ( $loop->have_posts() ) : $loop->the_post();
    $foo = get_post_meta($post->ID, 'Foo');
    print_r($foo);
endwhile;
?>