category-parts.php

//get category id 
$cat_id = get_cat_id(single_cat_title("",false));

$featured_IDs = array();

//get the three manual IDs
for($i = 0; $i < 3; $i++){
	$field_name = 'featured_post_id_'.($i+1);
	$feat_ID = get_field($field_name,'category_'.$cat_id);
	if($feat_ID && (intval($feat_ID) > 0) && (get_post_status($feat_ID) == 'publish')){
		array_push($featured_IDs,intval($feat_ID));
	}
}

//remove possible duplicates
$featured_IDs = array_values(array_unique($featured_IDs));	

//if didn't get three elements, add random featured posts of the same category
if(count($featured_IDs) < 3){
	$my_array = array(
		'post_type' => 'post',
		'post_status' => 'publish',
		'cat' => $cat_id,	
		'meta_key' => 'is_featured',
		'post__not_in' => $featured_IDs,
		'meta_value' =>	'1',
		'orderby' => 'rand',
		'posts_per_page' => 3,
		'fields' => 'ids',
	);
	$loop = new WP_Query($my_array);
	$randpost_IDs = $loop->posts;	

	//combine IDs
	$num_1 = count($featured_IDs);
	for($i = 0; $i < min((3 - $num_1),count($randpost_IDs)); $i++){
		array_push($featured_IDs, $randpost_IDs[$i]);
	}
}

//get posts
$featured_posts = empty($featured_IDs)? array()  :  array_map("get_post",$featured_IDs);