Remove the CSS code above then add this on a child-theme functions.php
add_action( 'wp_footer', 'ra_same_height_script' );
function ra_same_height_script() {
?>
<script>
jQuery(document).ready(function($) {
var height = Math.max($("#content").height(), $("#sidebar-left").height(), $("#sidebar-right").height());
$("#content").height(height);
$("#sidebar-left").height(height);
$("#sidebar-right").height(height);
});
</script>
<?php
}