Print Menu block Title and the menu
Your implementation is right but you are missing the print subject, you only print the content of the block.

if you debug the $block variable you will find 'subject' as key with value or without value based on the block if it has a title or not.

So your code should be like this in order to print the title.

 $block = module_invoke('menu_block', 'block_view', '3');
 print render($block['subject']);
 print render($block['content']);