There isn’t any way to add a value to the relationship field. Using just the relationship field you could only do this based on the count of how many fields are shown and show the first 3 differently
$count = 0;
foreach ($featured_posts as $post) {
$count++;
if ($count <=3) {
// one of the first 3 posts
} else {
// not one of the first 3 posts
}
}