New containing div after every 3 records

As tadman stated in the comment under your question. The best approach should use a modulus operator (%) with 3.

Place your separating condition at the start of each iteration. (Demo)

Like this:

$x=0;  // I prefer to increment starting from zero.
       // This way I can use the same method inside a foreach loop on
       // zero-indexed arrays, leveraging the keys, and omit the `++` line.
echo "<div class=\"row\">";
    foreach($rows as $row){
        if($x!=0 && $x%3==0){  // if not first iteration and iteration divided by 3 has no remainder...
            echo "</div>\n<div class="row">";
        }
        echo "<div>$row</div>";
        ++$x;
    }
echo "</div>";

This will create:

<div class="row"><div>one</div><div>two</div><div>three</div></div>
<div class="row"><div>four</div><div>five</div><div>six</div></div>

---

Late Edit, here are a couple of other methods for similar situations which will provide the same result:

foreach(array_chunk($rows,3) as $a){
    echo "<div class=\"row\"><div>",implode('</div><div>',$a),"</div></div>\n";
}

or

foreach ($rows as $i=>$v){
    if($i%3==0){
        if($i!=0){
            echo "</div>\n";
        }
        echo "<div class=\"row\">";
    }
    echo "<div>$v</div>";
}
echo "</div>";

---

To clarify what NOT to do…

Sinan Ulker’s answer will lead to an unwanted result depending on the size of your result array.

Here is a generalized example to expose the issue:

Using this input array to represent your pdo results:

$rows=["one","two","three","four","five","six"];

Sinan’s condition at the end of each iteration:

$i=1;
echo "<div class=\"row\">";
    foreach($rows as $row){
        echo "<div>$row</div>";
        if($i%3==0)echo "</div>\n<div class="row">";  // 6%3==0 and that's not good here
        // 6%3==0 and will echo the close/open line after the content to create an empty, unwanted dom element
        $i++;
    }
echo "</div>\n\n";

Will create this:

<div class="row"><div>one</div><div>two</div><div>three</div></div>
<div class="row"><div>four</div><div>five</div><div>six</div></div>
<div class="row"></div>                          //<--- this extra element is not good