add-class-to-first-3-posts.php

<?php
// This code is UNTESTED!
function foo_add_class_to_first_three_posts_or_whatever( $post_class ) {
	global $wp_query;

	if ( $wp_query && $wp_query->current_post < 3 && is_main_query() && ! is_singular() && ! is_search() ) {
		$post_class .= ' one-of-the-top-trio';
	}

	return $post_class;
}
add_filter( 'post_class', 'foo_add_class_to_first_three_posts_or_whatever' );