How to use multiple Meta Field from CPT as Post permalink

only the first meta field is used

Yes, and it’s because the $new_slug contains just one meta which is maker.

So if you want to add other meta like model and year, and have a slug that looks like <maker>-<model>-<year>, you can try my code below — just replace the $new_slug = get_post_meta($post_id,'maker', true); in your code with the following:

Note: You should use the first snippet in the question.

// List of meta that will be added to the post slug (post_name). Just add another
// get_post_meta() if you want to add another meta to the slug. And just reposition
// the array items based on your preferences, e.g. you could put the 'year' before
// 'maker'.
$meta = array(
    get_post_meta( $post_id, 'maker', true ),
    get_post_meta( $post_id, 'model', true ),
    get_post_meta( $post_id, 'year', true ),
);

$new_slug = implode( '-', array_filter( $meta ) );

target only my CPT “cars”

You could use get_post_type() like so:

add_action( 'save_post', 'set_slug' );
function set_slug( $post_id ) {
    // Do nothing if the post type is NOT "cars".
    if ( 'cars' !== get_post_type( $post_id ) ) {
        return;
    }

    // ... your code here.
}

But a simpler option is using the save_post_<post type> hook instead of save_post like so:

add_action( 'save_post_cars', 'set_slug' );
function set_slug( $post_id ) {
    // ... your code here.
}

Additional Notes

1. Just remove the $post_type = get_post_type($post,'cars', true); from your code.

2. Before you update the post, you should ensure that the slug is not already the same as the value of the $new_slug variable. So for example, add this after the above $new_slug = implode( '-', array_filter( $meta ) );:

   if ( strtolower( $new_slug ) === get_post_field( 'post_name', $post_id ) ) {
       return;
   }
   

3. “set_slug” is a very generic name, so please use a better name that is unique enough, e.g. wpse_387941_set_slug.. ( and be sure to update the function name in the remove_action() and add_action() calls ).