<div class="row">
$ergebnis = $mysqli->query("SELECT name FROM pages Where city = '1';");

while($zeile = $ergebnis->fetch_array()) {


echo "<div class="col-sm-4 col-md-3">
echo "<h3>".$zeile['name']."</h3>";
..
echo "</div>";

$i=0;
i++;
if ($i == 4){
echo "</div>";}

}
?>

<div class="row">
$ergebnis = $mysqli->query("SELECT name FROM pages Where city = '1';");

$i=0;
while($zeile = $ergebnis->fetch_array()) {
      echo "<div class="col-sm-4 col-md-3">
      echo "<h3>".$zeile['name']."</h3>";
      ..
      echo "</div>";

     $i++;
     if ($i == 4){
          echo "</div>";
          $i = 0;
     }
}
?>

<?php

    $ergebnis = $mysqli->query("SELECT name FROM pages Where city = '1';");

    $i=0;

    while($zeile = $ergebnis->fetch_array()) {

        if ($i == 0 || $i%4==0){ // <div class="row"> opens on entry and after every fourth entry
            echo "<div class="row">";
        }

        echo "<div class="col-sm-4 col-md-3">";
        echo "<h3>".$zeile['name']."</h3>";
        /*
            Rest of your code
        */
        echo "</div>";

        $i++;
        if ($i % 3 == 0){
            echo "</div>"; // <div class="row"> closes here on every fourth entry
        }

    }
?>