How to insert element into arrays at specific position?

Let’s imagine that we have two arrays:

$array_1 = array(
  '0' => 'zero',
  '1' => 'one',
  '2' => 'two',
  '3' => 'three',
);

$array_2 = array(
  'zero'  => '0',
  'one'   => '1',
  'two'   => '2',
  'three' => '3',
);

Now, I’d like to insert array('sample_key' => 'sample_value') after third element of each array. How can I do it?