dantula,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function(){
var insert_after = 3, block = $(".myadsense:first");
$(".article br").each((i, el) => {
if(i && (i % insert_after) == 0) $(el).after(block.clone());
});
block.remove();
});
</script>
</head>
<body>
<div class="myadsense">
... код Adsense ...
</div>
<div class="article">
<br>01
<br>02
<br>03
<br>04
<br>05
<br>06
<br>07
<br>08
<br>09
<br>10</div>
</body>
</html>