WP categories on home (blog) page

gistfile1.aw

<?php

function category_homepage( $query ) {
	if ( $GLOBALS['wp_the_query'] != $query || ! $query->is_home() ) // Only filter the main query on the blog page
		return;

	$categories = array( 1, 2, 3 );

	$query->set('category__in', $categories);
}

add_action( 'pre_get_posts', 'category_homepage' );