LEFT JOIN sur trois tables

$sql = "SELECT * FROM mr_matnivclts LEFT 
JOIN mr_matieres ON mr_matieres.matiere_id = mr_matnivclts.matnivclt_id WHERE client_id = '$client_id'
JOIN mr_niveaux ON mr_niveaux.niveau_id = mr_matnivclts.matnivclt_id WHERE client_id = '$client_id'";

$sql = "SELECT * 
FROM mr_matnivclts MM
LEFT JOIN mr_matieres MA ON MA.matiere_id = MM.matnivclt_id 
LEFT JOIN mr_niveaux N ON N.niveau_id = MM.matnivclt_id 
WHERE MM.client_id = '$client_id'";

$sql = "SELECT * FROM mr_matnivclts 
LEFT JOIN mr_matieres ON mr_matieres.matiere_id 
LEFT JOIN mr_niveaux ON mr_niveaux.niveau_id = mr_matnivclts.matnivclt_id WHERE client_id = '$client_id'";       
$requete1 = $bdd->query($sql);
while( $result = mysqli_fetch_object($requete1) )
  {
              <?php    echo(
           "<div align=\"center\">".$result->ville_id."------".$result->ville."------".
$result->cp." <a href=\"modifier_intervenant3.php?matniv_id=".
$result-> ville_id."\">modifier--- ) ;

          }

		 $sql = "SELECT *
	      FROM mr_matnivclts
	      LEFT JOIN mr_matieres ON mr_matieres.matiere_id = mr_matnivclts.matiere_id
	      LEFT JOIN mr_niveaux ON mr_niveaux.niveau_id = mr_matnivclts.niveau_id
	      WHERE client_id = '$client_id'";
	      

	//exécution de la requête:
 	$requete1 = $bdd->query($sql);

    //affichage des données:
    while( $result = mysqli_fetch_object($requete1) )
    {
?>
    

					 <?php    echo(
           "<div align=\"center\">".$result->matiere_id."------".$result->matiere."------".$result->niveau." </div>\n"
       ) ;