Need to get specific data from array

You should be able to rewrite what you have about and also use get_permalink as @Benoti stated while omitting the $split array.

get_permalink accepts either the Post ID or a post object.

global $rental;

$images = get_children( array(
    'post_parent' => $rental_id,
    'post_status' => 'inherit',
    'post_type' => 'attachment',
    'post_mime_type' => 'image',
    'order' => 'ASC',
    'orderby' => 'menu_order ID'
) );

$array = $images;
$number_of_elements = 3;
$count = count( $array );

for ( $i = 0; $i <= $count - 1; $i++ ) {
    $slices = array_slice( $array, $i , $number_of_elements);
    if ( count( $slices ) != $number_of_elements )
        break;

    echo "<div class="Outer Top">";
    foreach( $slices as $inneritem ) {
        $link = wp_get_attachment_url( $inneritem->ID );
        echo "<div class="Inner Top">";
        echo "<img src="https://wordpress.stackexchange.com/questions/251866/$link">";
        echo "</div>";
    }
    echo "</div>";
}