Php mysql arrange html output three by three

I think you are looking for an algorithm similar to this. It will add an open <div> at 1 and close <div> at 3. You would have to accommodate for numbers that leave a remainder:

// Set the start at 1
$i = 1;
foreach ($sql as $row){
    // If the loop hits 1, add <div>
    if($i == 1)
        echo '<div>'.PHP_EOL;
    echo $id = "\t<li>".$row["post_id"].'</li>'.PHP_EOL;
    // If the loop hits 3, add </div>
    if($i == 3) {
        echo '</div>'.PHP_EOL;
        // Reset the counter
        $i = 0;
    }
    // Increment up
    $i++;
}

With random numbers will give you something like:

<div>
        <li>309781723</li>
        <li>1591425966</li>
        <li>1922824365</li>
</div>
<div>
        <li>1861766038</li>
        <li>2065624155</li>
        <li>504061231</li>
</div>
<div>
        <li>143488299</li>
        <li>806735480</li>
        <li>605627018</li>
</div>
<div>
        <li>1305718337</li>
        <li>2080669131</li>
        <li>314748482</li>
</div>

Here is another example: Displaying data from arrays in a dynamic table PHP

You can also do an array_chunk() similar to this. You don't need any calculations to accommodate for remainders in this scenario:

// Split into sets of 3
$arr    =   array_chunk($sql,3);
$a = 0;
// You could probably do an array_map() but I am not experienced enough
// with array_map() to do it efficiently...or even at all, to be perfectly honest
foreach($arr as $row) {
        for($i = 0; $i < count($row); $i++)
            $new[$a][]  =   $row[$i]['post_id'];
        // Assemble the rows via implode
        $nRow[] =   "\t<li>".implode("</li>".PHP_EOL."\t<li>",$new[$a])."</li>".PHP_EOL;

        $a++;
    }
// Implode with <div> wrappers
echo "<div>".PHP_EOL.implode("</div>".PHP_EOL."<div>".PHP_EOL,$nRow)."</div>";