[js] referenziare una select multiple

[ganesha]

Ho una pagina di questo tipo:

<form name=frm action=inserisci.php method=post onsubmit="return checkdata();">
<table>
 <tr>
  <td>
   <select name=var1>
     <option value=1>text1</option>
     <option value=2>text2</option>
   </select>
  </td>
  <td>
   <select name=var2[] multiple>
     <option value=1>text1</option>
     <option value=2>text2</option>
   </select>
  </td>
  <td>
   <select name=var3[] multiple>
     <option value=1>text1</option>
     <option value=2>text2</option>
   </select>
  </td>
 </tr>
</table>
<input type=submit value=Inserisci>
</form>

<script language=javascript>
function checkdata(){
  v1=document.frm.var1.value;

  ...

  return esito;
}
</script>

come faccio, nella funzione checkdata(), ad accedere alle select var2 e var3?