Código PHP:
$sql = "SELECT * FROM materias_docentes A join materias as B ON (A.materia=B.idmat) join carreras C ON (B.carrera=C.codigo) where A.codigo='63531273'";
$result = mysql_query($sql,$conex);
Código PHP:
while($row = mysql_fetch_array($result)){
echo ' <tr>
<td>'.$row['A.nombre'].'</td>
<td>'.$row['B.nombre'].'</td>
</tr>';
}