Why not just get the count for each post type and sum them?
function get_all_them_ven_posts(){
$count= 0;
$post_types = [ 'postType1', 'postType2', 'postType3' ];
foreach( $post_types as $post_type ) {
$count_posts = wp_count_posts( $post_type );
$count = $count + $count_posts->publish;
}
return $count;
}