add_filter('the_content', 'prefix_insert_post_ads');

function prefix_insert_post_ads($content) {

 $insertion = 'put your adsense code here';

 if (is_single() && ! is_admin()) {
  return prefix_insert_after_paragraphs($content, $insertion, array(3));
 }

 return $content;

}

function prefix_insert_after_paragraphs($content, $insertion, $paragraph_indexes) {

 preg_match_all('#</p>#i', $content, $matches, PREG_SET_ORDER+PREG_OFFSET_CAPTURE);

 $matches = array_map(function($match) {
  return $match[0][1] + 4; // return string offset + length of </p> Tag
 }, $matches);

 rsort($paragraph_indexes); 

 foreach ($paragraph_indexes as $paragraph_index) {
  if ($paragraph_index <= count($matches)) {
   $offset_position = $matches[$paragraph_index-1];
   $content = substr($content, 0, $offset_position) . $insertion . substr($content, $offset_position);
  }
 }

 return $content;

}