WordPress: Add extra post types to main WP post loop. Add this code to functions.php file.

add_extra_post_type_to_loop.php

add_filter( 'pre_get_posts', 'my_get_posts' );

function my_get_posts( $query ) {

	if ( is_home() && $query->is_main_query() ) {
		$query->set( 'post_type', array( 'post', 'custom_post_name' ) );
		$query->set( 'order', 'ASC');
		$query->set( 'posts_per_page', 3);
	}

	return $query;
}